drm: Reshuffle getfb error returns

Make it a little more clear what's going on inside of getfb, and also
make it easier to add alternate paths to get a handle in future.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180323134553.15993-3-daniels@collabora.com