tests/gem_exec_blt: More reasonable runtime
tests/gem_exec_blt: More reasonable runtime

Doing (1 << 18) - 1 repeats isn't really. With the reduced time the
test is also useful in some shell scripted tests which need some real
load (but not too much), like drv_missed_irq_hang.

Signed-off-by: Daniel Vetter <c1b6782c4af8f0673da8923a0702a1832e5940f4@ffwll.ch>

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