drm/i915/bios: assume vbt is 4-byte aligned into oprom

The unaligned ioread32() will make us read byte by byte looking for the
vbt. We could just as well have done a ioread8() + a shift and avoid the
extra confusion on how we are looking for "$VBT".

However when using ACPI it's guaranteed the VBT is 4-byte aligned
per spec, so we can probably assume it here as well.

v2: do not try to simplify the loop by eliminating the auxiliary counter
(Jani and Ville)
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191126225110.8127-4-lucas.demarchi@intel.com

drivers/gpu/drm/i915/display/intel_bios.c

@@ -1938,7 +1938,7 @@ static struct vbt_header *oprom_get_vbt(struct drm_i915_private *dev_priv)
 		return NULL;
 
 	/* Scour memory looking for the VBT signature. */
-	for (i = 0; i + 4 < size; i++) {
+	for (i = 0; i + 4 < size; i += 4) {
 		if (ioread32(oprom + i) != *((const u32 *)"$VBT"))
 			continue;