Skip MCHBAR queries when display is not available

Platforms without display do not map the MCHBAR MMIO into the GFX
device BAR. Skip this sequence when display is not available.

Signed-off-by: Stuart Summers <stuart.summers@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191120004505.149516-1-stuart.summers@intel.com