Runtime: disable some unecessary image formats.

Runtime: disable some unecessary image formats.

Per OpenCL, the minimum list of supported format is as below:
CL_RGBA:
  CL_UNORM_INT8
  CL_UNORM_INT16
  CL_SIGNED_INT8
  CL_SIGNED_INT16
  CL_SIGNED_INT32
  CL_UNSIGNED_INT8
  CL_UNSIGNED_INT16
  CL_UNSIGNED_INT32
  CL_HALF_FLOAT
  CL_FLOAT

CL_BGRA:
  CL_UNORM_INT8

Let's only support this type and CL_R currently.

Also removed an unnecessary assertion. And fix the CL_Rx's type size.

Signed-off-by: Zhigang Gong <zhigang.gong@linux.intel.com>
Reviewed-by: He Junyan <junyan.he@inbox.com>