drm/i915/perf: check the value of PROP_SAMPLE_OA uapi parameter

We've been a bit loose about this opening parameter. We should only
add the flag for writing OA reports when the value of this parameter
is != 0.

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180326090831.22686-3-lionel.g.landwerlin@intel.com