do not call memcpy for cl_enqueue_read_buffer if userptr is enabled
do not call memcpy for cl_enqueue_read_buffer if userptr is enabled

sometimes, application invokes read buffer, instead of map buffer,
even if userptr is enabled. memcpy is not necessary for such case.

Signed-off-by: Guo Yejun <1f883e24fa2949df61a83c432caa290810cece11@intel.com>
Reviewed-by: Yang Rong <78759e37dc7114caed18f74e7515c5cb519dbf6d@intel.com>

wdv4758h/beignet,wdv4758h/beignet,wdv4758h/beignet,wdv4758h/beignet,wdv4758h/beignet