drm/xe: Don't use __user error pointers

The error pointer macros are not aware of __user pointers and as a
consequence sparse warns.

Have the copy_mask() function return an integer instead of a __user
pointer.

Fixes: dd08ebf6 ("drm/xe: Introduce a new DRM driver for Intel GPUs")
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240117134048.165425-5-thomas.hellstrom@linux.intel.com