drm: Fix double free from checking if gamma lut has been updated

The code falls through to "fail" under all conditions, so there is no
need for the drm_property_blob_put if the gamma lut hasn't been changed.
Fixes: 9cca2667 "drm: Check whether the gamma lut has changed before updating"

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>