drm/i915: add dev.i915.oa_max_sample_rate sysctl

The maximum OA sampling frequency is now configurable via a
dev.i915.oa_max_sample_rate sysctl parameter.

Following the precedent set by perf's similar
kernel.perf_event_max_sample_rate the default maximum rate is 100000Hz

Signed-off-by: Robert Bragg <robert@sixbynine.org>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Sourab Gupta <sourab.gupta@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20161107194957.3385-10-robert@sixbynine.org