Don't try to access i915-perf data if no file was given
Don't try to access i915-perf data if no file was given

With USE_I915_PERF=1, if the user opens a .dat file without a
.i915-dat one, we run into a segfault... Oopsie

mikesart/gpuvis,mikesart/gpuvis,mikesart/gpuvis