drm: Improve on minor type helpers v3

Add a drm_is_legacy() helper, constify argument to drm_is_render_client(),
and use / change helpers where appropriate.

v2: s/drm_is_legacy/drm_is_legacy_client/ and adapt to new code context.
v3: s/legacy_client/primary_client/

Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Brian Paul <brianp@vmware.com>

diff --cc drivers/gpu/drm/drm_crtc.c
Simple merge

diff --cc drivers/gpu/drm/drm_fops.c
Simple merge

diff --cc include/drm/drmP.h
Simple merge