testdisplay: Properly handle the life cycle of framebuffers
testdisplay: Properly handle the life cycle of framebuffers

When cycling through the modes, let's make sure to free the previous
framebuffers. This is the perfect occasion to use kmstest_remove_fb().

Signed-off-by: Damien Lespiau <64bd3cb94f359c1a3ce68dae5e26b40578526277@intel.com>

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