tests/gem_reg_read: actually wait a bit between reading TIMESTAMP reg
tests/gem_reg_read: actually wait a bit between reading TIMESTAMP reg

QA could only hit this on ilk because the timer increase quite a bit
slower on that platform than on gen6+ (80ns vs 320ns, iirc).

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=52937

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