drm/dsc: Fix bogus cpu_to_be16() usage

__be16 = cpu_to_be16(__be16) is nonsense. Do it right.

../drivers/gpu/drm/drm_dsc.c:218:53: warning: incorrect type in assignment (different base types)
../drivers/gpu/drm/drm_dsc.c:218:53:    expected restricted __be16
../drivers/gpu/drm/drm_dsc.c:218:53:    got int
../drivers/gpu/drm/drm_dsc.c:225:25: warning: cast from restricted __be16
../drivers/gpu/drm/drm_dsc.c:225:25: warning: incorrect type in argument 1 (different base types)
../drivers/gpu/drm/drm_dsc.c:225:25:    expected unsigned short [usertype] val
../drivers/gpu/drm/drm_dsc.c:225:25:    got restricted __be16
../drivers/gpu/drm/drm_dsc.c:225:25: warning: cast from restricted __be16
../drivers/gpu/drm/drm_dsc.c:225:25: warning: cast from restricted __be16

Cc: Manasi Navare <manasi.d.navare@intel.com>
Cc: David Francis <David.Francis@amd.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190710125143.9965-2-ville.syrjala@linux.intel.com
Reviewed-by: Sean Paul <sean@poorly.run>