drm: add 'count' to struct drm_object_properties

This way, we don't need to count every time, so we're a little bit
faster and code is a little bit smaller.

Change suggested by Ville Syrjälä.

Reviewed-by: Rob Clark <rob.clark@linaro.org>
Tested-by: Rob Clark <rob.clark@linaro.org>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>