drm/i915/bios: Extract intel_spi_read16()

The SPI VBT codepath only knows how to read 4 bytes at a time.
So to read the 2 byte vbt_size it masks out the unwanted msbs.
Hide that little implementation detail inside a new intel_spi_read16()
helper. Alse rename the existing intel_spi_read() to intel_spi_read32()
to make it clear what it does.

Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240923152453.11230-5-ville.syrjala@linux.intel.com