drm/msm: Clean up split driver features

Avoid the possibility of missing features between the split and unified
drm driver cases by defining DRIVER_FEATURES_GPU / KMS and using those
in the drm_driver initializations.

Signed-off-by: Rob Clark <robin.clark@oss.qualcomm.com>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>
Patchwork: https://patchwork.freedesktop.org/patch/662595/