drm/i915/bios: Round PCI ROM VBT allocation to multiple of 4

The SPI code rounds the VBT allocation to a multiple of four bytes
(presumably because it reads the VBT 4 bytes at a time). Do the
same for the PCI ROM side to eliminate pointless differences between
the two codepaths. This will make no functional difference.

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/20240923152453.11230-4-ville.syrjala@linux.intel.com