drm/i915: add back GEN12_BDSM_MASK

The mask was added in commit e5f415bf ("drm/i915: Add missing mask
when reading GEN12_DSMBASE"), but then looks to be dropped in some
unrelated code movement in commit dbb2ffbf ("drm/i915/mtl: enable
local stolen memory") without explanation. Add it back.

Fixes: dbb2ffbf ("drm/i915/mtl: enable local stolen memory")
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Aravind Iddamsetty <aravind.iddamsetty@intel.com>
Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Cc: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221005153148.758822-1-matthew.auld@intel.com

drivers/gpu/drm/i915/gem/i915_gem_stolen.c

@@ -910,7 +910,7 @@ i915_gem_stolen_lmem_setup(struct drm_i915_private *i915, u16 type,
 		GEM_BUG_ON((dsm_size + SZ_8M) > lmem_size);
 	} else {
 		/* Use DSM base address instead for stolen memory */
-		dsm_base = intel_uncore_read64(uncore, GEN12_DSMBASE);
+		dsm_base = intel_uncore_read64(uncore, GEN12_DSMBASE) & GEN12_BDSM_MASK;
 		if (WARN_ON(lmem_size < dsm_base))
 			return ERR_PTR(-ENODEV);
 		dsm_size = lmem_size - dsm_base;