drm/i915: use backlight legacy combination mode also for i915gm/i945gm

i915gm and i945gm also seem to use and need the legacy combination mode
bit in BLC_PWM_CTL.

v2: Also do this for i915gm (Ville).

Reported-and-tested-by: Luis Ortega <luiorpe1@gmail.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=75001


Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>