drm/i915/bios: Add some size checks to SPI VBT read

Unify the SPI vs. PCI ROM VBT read codepaths a bit by
pulling some size overflow checks from the PCI side
into the SPI side.

v2: s/drm_dbg()/drm_dbg_kms()/

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/20240923152453.11230-3-ville.syrjala@linux.intel.com