drm/i915: Module parameters using '-1' as default must be signed type

Testing i915_panel_use_ssc for the default value was broken, so the
driver would never autodetect the correct value.

Signed-off-by: Keith Packard <keithp@keithp.com>
Reviewed-by: Michel Alexandre Salim <salimma@fedoraproject.org>
Tested-by: Michel Alexandre Salim <salimma@fedoraproject.org>
Cc: stable@kernel.org