drm/xe/display: fix i915_gem_object_is_shmem() wrapper

shmem ensures the memory is cleared on allocation, however here we are
using TTM, which doesn't natively support shmem (other than for swap),
but instead just allocates normal system memory. And we only zero such
memory for userspace allocations. In the case of intel_fbdev we are
missing the memset_io() since display path incorrectly thinks object is
shmem based.

Fixes: 44e69495 ("drm/xe/display: Implement display support")
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Suraj Kandpal <suraj.kandpal@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240205153110.38340-2-matthew.auld@intel.com