drm: Get atomic property value even if DRIVER_ATOMIC is not set

drm: Get atomic property value even if DRIVER_ATOMIC is not set

i915 does not set DRIVER_ATOMIC by default yet but uses atomic_check and
atomic_commit. drm_object_property_get_value() does not read the correct
value of atomic properties if DRIVER_ATOMIC is not set. Checking whether
the driver uses atomic modeset is a better check instead as the property
values are tracked in the state structures.

v2: Included header

Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1482396643-32456-2-git-send-email-dhinakaran.pandiyan@intel.com