drm/i915/vga: Don't touch VGA registers if VGA decode is fully disabled

drm/i915/vga: Don't touch VGA registers if VGA decode is fully disabled

On some systems the BIOS will disable the VGA decode logic in the
iGPU (via GMCH_CTRL) when an external GPU is used as the primary
VGA device. In that case the iGPU will never claim any VGA register
accesses, and any access we do will in fact end up on the external
GPU. Don't go poking around in the other GPUs' registers.

Note that (at least on the g4x board where I tested this) the BIOS
forgets to set the VGACNTR VGA_DISP_DISABLE bit, and the reset
value for said bit is 0. That apparently prevents the pipes from
running, so we must still remember to set the bit, despite the VGA
plane was never actually enabled. On more modern platforms (hsw+
maybe?) the reset value for VGACNTR was changed to have
VGA_DISP_DISABLE already set.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patch.msgid.link/20251208182637.334-6-ville.syrjala@linux.intel.com
Acked-by: Jani Nikula <jani.nikula@intel.com>