igt/gem_concurrent_blit: Exercise concurrent reads
igt/gem_concurrent_blit: Exercise concurrent reads

Since we are considering implementing read-read optimisations for mixed
engine workloads, make sure that we at least have a few tests that check
for coherency when doing so.

Signed-off-by: Chris Wilson <711c73f64afdce07b7e38039a96d2224209e9a6c@chris-wilson.co.uk>

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