drm: Dust off module build a little.

drm: Sync (device-independent) drmkms*.kmod makefiles.

Rename fence -> dma_fence, step 2: files files.

Bump default value of WARNS for modules from 3 to 5, and
explicitly set WARNS for modules that fail with WARNS=5.

Also, turn on -Wno-missing-noreturn for clang for some files.

At the moment, among ~ 360 modules,
- 2 (lua and zfs) need WARNS=0
- 1 (solaris) needs WARNS=1
- 136 need WARNS=3 (mostly due to sign-compare)
- 4 need WARNS=4
- others can be compiled with WARNS=5

Discussed on tech-kern.

Rework PCI and AGP conditionalization.

- Push drm_pci_set_unique into driver callback.
- Eliminate drm_pci_set_unique_hook.
- Gather all drm_agp_* functions into struct drm_agp_hooks.
- Replace the nonsensical old atomic garbage by serious locking.
- Make drm_agpsupport.c its own module.
- Eliminate NDRMKMS_PCI.
- Use NAGP from "agp.h" only in drm_module.c for horrible hack.
=> See comment in file for rationale.
- Always define CONFIG_PCI=1 and CONFIG_AGP=1.
- Always go through the drm_agp_* function hooks.
- Ifdef out nouveau agp stuff that doesn't go through drm_agp_*
for reasons that I'm too frustrated to figure out tonight.
- pci_iomap no longer automagically does agp_i810_borrow.
=> Use drm_agp_borrow instead.

Move the Linux PCI shim into its very own .c file.

The poor thing has deserved this for a very long time, even though it
is full of kludges. Also I'm tired of recompiling everything every
time I touch it.

Add some components to drmkms_linux.kmod.

Sort.

branches: 1.7.16; 1.7.18;
Move linux_work.c and workqueue.h from sys/external/bsd/drm2 to
sys/external/common so that they can be used by others.

LGTM from riastradh@

These should really live outside sys/external, but that can be decided
later

Move Linux ww_mutex code into a .c file where it belongs.

branches: 1.5.2;
Fix module build of drmkms_linux.

branches: 1.4.2; 1.4.4;
Remove needless linux_gfp.c.

Trying to make do with Linux's page allocation interface is pretty
much always wrong: either we let uvm allocate pages in uvm objects
for us, or we use bus_dma.

Make it build and boot on my test machines.

Screen blanks on boot on the Ivy Bridge system with

DRM error in cpt_serr_int_handler: PCH transcoder A FIFO underrun

But after that everything is OK. Appears to be an upstream problem.
To investigate...

I think there's a cache flushing issue somewhere -- there are little
display artefacts on my T60.

branches: 1.2.2; 1.2.4; 1.2.6;
Merge riastradh-drm2 to HEAD.

branches: 1.1.2;
file Makefile was initially added on branch riastradh-drm2.