vrend/decode: refactor parameters check in clear_texture
Currently virgl is not angry with commands, but I'm not getting expect clear effect.
94%: Checks: 19, Failures: 1, Errors: 0
../virglrenderer/tests/test_virgl_cmd.c:1455:F:clear:virgl_test_clear_texture:0: Assertion 'ptr[i] == test_green' failed: ptr[i] == 0, test_green == 4278255360