drm/i915: Add missing mask when reading GEN12_DSMBASE

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 <[email protected]>
Reviewed-by: Caz Yokoyama <[email protected]>
Reviewed-by: Wayne Boyer <[email protected]>
Signed-off-by: Lucas De Marchi <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]