drm/i915/vga: Introduce intel_vga_{read,write}()

drm/i915/vga: Introduce intel_vga_{read,write}()

VGA register are rather special since they either get accessed
via the global IO addresses, or possibly through MMIO on
pre-g4x platforms. Wrap all VGA register accesses in
intel_vga_{read,write}() to make it obvious where they get
accessed.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patch.msgid.link/20251208182637.334-13-ville.syrjala@linux.intel.com
Reviewed-by: Jani Nikula <jani.nikula@intel.com>