drm: fix a typo in a comment

Spotted while reviewing the DRM changes in Linux 3.18 for LinuxFR.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Martin Peres <martin.peres@free.fr>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>