drm: add __user attribute to ptr_to_compat()

The ptr_to_compat() call takes a "void __user *", so cast
the compat drm calls that use it to avoid the following
warnings from sparse:

drivers/gpu/drm/drm_ioc32.c:188:39: warning: incorrect type in argument 1 (different address spaces)
drivers/gpu/drm/drm_ioc32.c:188:39:    expected void [noderef] <asn:1>*uptr
drivers/gpu/drm/drm_ioc32.c:188:39:    got void *[addressable] [assigned] handle
drivers/gpu/drm/drm_ioc32.c:529:41: warning: incorrect type in argument 1 (different address spaces)
drivers/gpu/drm/drm_ioc32.c:529:41:    expected void [noderef] <asn:1>*uptr
drivers/gpu/drm/drm_ioc32.c:529:41:    got void *[addressable] [assigned] handle

Cc: stable@vger.kernel.org
Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190301120046.26961-1-ben.dooks@codethink.co.uk