drm/i915/dpcd_bl: Unbreak enable_dpcd_backlight modparam

Looks like I accidentally made it so you couldn't force DPCD backlight
support on, whoops. Fix that.

Signed-off-by: Lyude Paul <lyude@redhat.com>
Fixes: 17f5d579

 ("drm/i915: Force DPCD backlight mode on X1 Extreme 2nd Gen 4K AMOLED panel")
Cc: Adam Jackson <ajax@redhat.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: "Ville Syrjälä" <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200413214407.1851002-1-lyude@redhat.com
(cherry picked from commit d7fb38ae

)
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>