Code review comment for lp:~andreas-pokorny/mir/fix-1391975

drmGetCap doesn't seem to be any more defensive than simply calling gbm_bo_get_width/height. In fact it might be more error-prone to risk requesting something that's not 64x64 for other drivers. So we should ensure we're always asking for 64x64 right now, but also be careful to query what we're actually given:
   gbm_bo_get_width
   gbm_bo_get_height