drm/i915/dg1: Read OPROM via SPI controller

Read OPROM SPI through MMIO and find VBT entry since we can't use
OpRegion and PCI mapping may not work on some systems due to most BIOSes
not leaving the Option ROM mapped.

v2: Remove message with allocation failure

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: Clint Taylor <clinton.a.taylor@intel.com>
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Acked-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211216062645.3477854-1-lucas.demarchi@intel.com