drm/i915/vga: Clean up VGA registers even if VGA plane is disabled

drm/i915/vga: Clean up VGA registers even if VGA plane is disabled

Turns out at least some systems (eg. HSW Lenovo ThinkCentre E73)
configure the VGA registers even when booting in UEFI mode. So
in order to avoid any issues with the MSR register we should
clean up the VGA registers anyway.

For now this mostly avoids the potential for unclaimed register
accesses due to the power well vs. MDA/CGA selection. But this
will become more important soon as we'll start to rely on the
MSR register to control VGA memory decode as well.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patch.msgid.link/20251208182637.334-7-ville.syrjala@linux.intel.com
Acked-by: Jani Nikula <jani.nikula@intel.com>