drm: split ati_pcigart.h out of drmP.h

Just move this into a separate header file, and make the
two users use it.

Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>