drm: convert drm context code to use Linux idr

This converts the drm context allocator to an idr, using the new idr
interface features from Kristian.

Fixes from Kristian Hoegsberg <krh@redhat.com>

Signed-off-by: Dave Airlie <airlied@linux.ie>

diff --cc drivers/char/drm/drmP.h
Simple merge

diff --cc drivers/char/drm/drm_context.c
Simple merge