drm: Extract drm_drv.h

I want to move dumb buffer documentation into the right vfuncs, and
for that I first need to be able to pull that into kerneldoc without
having to clean up all of drmP.h. Also, header-splitting is nice.

While at it shuffle all the function declarations for drm_drv.c into
the right spots, and drop the kerneldoc for drm_minor_acquire/release
since it's only used internally.

v2: Keep all existing copyright notices (Chris).

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>