drm/i915/bios: add helper for reading SPI

Add helper for reading SPI to not duplicate the write&read combo
everywhere.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230512111446.1524038-1-jani.nikula@intel.com