drm/i915/bios: set default backlight controller index

With backlight controller set to -1 in intel_panel_init_alloc() to
distinguish uninitialized values, and controller later being set only if
it's present in VBT, we can end up with -1 for the controller:

    [drm:intel_bios_init_panel [i915]] VBT backlight PWM modulation
    frequency 200 Hz, active high, min brightness 0, level 255,
    controller 4294967295

There's no harm if it happens on platforms that ignore controller due to
only one backlight controller being present, like on VLV above, but play
it safe.

Fixes: bf38bba3 ("drm/i915: Try to use the correct power sequencer intiially on bxt/glk")
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230207111626.1839645-1-jani.nikula@intel.com


(cherry picked from commit a0dcb06d)
Signed-off-by: Jani Nikula <jani.nikula@intel.com>