drm: fix some spelling mistakes

Signed-off-by: Frank Binns <frank.binns@imgtec.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1466763337-27428-1-git-send-email-frank.binns@imgtec.com