drm/i915/bios: gracefully disable dual eDP for now

For the time being, neither the power sequencer nor the backlight code
properly support two eDP panels simultaneously. While the software
states will be independent, the same sets of registers will be used for
both eDP panels, clobbering the hardware state and leading to errors.

Gracefully disable dual eDP until proper support has been added.

Cc: José Roberto de Souza <jose.souza@intel.com>
Cc: Uma Shankar <uma.shankar@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Swati Sharma <swati2.sharma@intel.com>
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>
Acked-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Tested-by: Swati Sharma <swati2.sharma@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211005175636.24669-1-jani.nikula@intel.com