drm/i915/dsb: Remove PIN_MAPPABLE from the DSB object VMA

It sounds like the hardware only needs the DSB object to be in global GTT
and not in the mappable region.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Animesh Manna <animesh.manna@intel.com>
Reviewed-by: Animesh Manna <animesh.manna@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191017155810.21654-1-tvrtko.ursulin@linux.intel.com