drm: Add a few missing descriptions in drm_driver docs

Fixes the following warnings when building docs:
../include/drm/drm_drv.h:553: warning: No description found for parameter 'debugfs_init'
../include/drm/drm_drv.h:553: warning: No description found for parameter 'gem_open_object'
../include/drm/drm_drv.h:553: warning: No description found for parameter 'gem_close_object'
../include/drm/drm_drv.h:553: warning: No description found for parameter 'prime_handle_to_fd'
../include/drm/drm_drv.h:553: warning: No description found for parameter 'prime_fd_to_handle'
../include/drm/drm_drv.h:553: warning: No description found for parameter 'gem_prime_export'
../include/drm/drm_drv.h:553: warning: No description found for parameter 'gem_prime_import'
../include/drm/drm_drv.h:553: warning: No description found for parameter 'gem_vm_ops'
../include/drm/drm_drv.h:553: warning: No description found for parameter 'major'
../include/drm/drm_drv.h:553: warning: No description found for parameter 'minor'
../include/drm/drm_drv.h:553: warning: No description found for parameter 'patchlevel'
../include/drm/drm_drv.h:553: warning: No description found for parameter 'name'
../include/drm/drm_drv.h:553: warning: No description found for parameter 'desc'
../include/drm/drm_drv.h:553: warning: No description found for parameter 'date'
../include/drm/drm_drv.h:553: warning: No description found for parameter 'driver_features'

There are still a couple more warnings for prime helpers that are
documented elsewhere.

Signed-off-by: Sean Paul <seanpaul@chromium.org>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170720174746.29100-5-seanpaul@chromium.org