benchmarks/gem_exec_ctx: Run for a fixed time
benchmarks/gem_exec_ctx: Run for a fixed time

Rather than investigate the curve for dispatch latency, just run for a
fixed time and report an average latency. Instead offer two modes,
average single dispatch latency, average continuous dispatch latency.

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

tiagovignatti/intel-gpu-tools,tiagovignatti/intel-gpu-tools,tiagovignatti/intel-gpu-tools,tiagovignatti/intel-gpu-tools