drm/i915/bios: fix slab-out-of-bounds access

drm/i915/bios: fix slab-out-of-bounds access

If VBT size is not a multiple of 4, the last 4-byte store will be out of
bounds of the allocated buffer. Spotted with KASAN. Round up the
allocation size.

v2: Use round_up() intead of roundup() as it's a power of 2 (Thomas)

Reported-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Fixes: a36e7dc0af1c ("drm/i915/dg1: Read OPROM via SPI controller")
Cc: Clint Taylor <clinton.a.taylor@intel.com>
Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211222081654.1843211-1-jani.nikula@intel.com