drm/i915: Add missing mask when reading GEN12_DSMBASE

DSMBASE register is defined so BDSM bitfield contains the bits 63 to 20
of the base address of stolen. For the supported platforms bits 0-19 are
zero but that may not be true in future. Add the missing mask.

v2: Use REG_GENMASK64()

Acked-by: Aravind Iddamsetty <aravind.iddamsetty@intel.com>
Reviewed-by: Caz Yokoyama <caz@caztech.com>
Reviewed-by: Wayne Boyer <wayne.boyer@intel.com>
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220915-stolen-v2-1-20ff797de047@intel.com