testdisplay: ignore invalid subtest options
testdisplay: ignore invalid subtest options

testdisplay does not have any subtests and should therefore exit with
the appropriate exit code if the --list-subtests or --run-subtest
options are used.

Signed-off-by: Thomas Wood <b9025d6a953b68f894a0f6a80fd2af6d55b0badd@intel.com>

chenxianqin/intel-gpu-tools,yipdw/intel-gpu-tools,dlespiau/checkmate-test-igt,mv0/intel-gpu-tools,mv0/intel-gpu-tools,tiagovignatti/intel-gpu-tools,mv0/intel-gpu-tools,yipdw/intel-gpu-tools,chenxianqin/intel-gpu-tools,erikarn/intel-gpu-tools,yipdw/intel-gpu-tools,chenxianqin/intel-gpu-tools,dlespiau/checkmate-test-igt,erikarn/intel-gpu-tools,tiagovignatti/intel-gpu-tools,erikarn/intel-gpu-tools,dlespiau/checkmate-test-igt,dlespiau/checkmate-test-igt,yipdw/intel-gpu-tools,tiagovignatti/intel-gpu-tools,mv0/intel-gpu-tools,tiagovignatti/intel-gpu-tools,chenxianqin/intel-gpu-tools,erikarn/intel-gpu-tools