drm/i915/bios: rename bios to oprom when mapping pci rom

oprom is actually a better name to use when using
pci_map_rom(). "bios"  is way too generic and confusing.

Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191108003602.33526-2-lucas.demarchi@intel.com