drm/i915/perf: remove user triggerable warn

Don't throw a warning if we are given an invalid property id. While
here let's also bring back Robert' original idea of catching unhandled
enumeration values at compile time.

Fixes: eec688e1 ("drm/i915: Add i915 perf infrastructure")
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Robert Bragg <robert@sixbynine.org>
Reviewed-by: Robert Bragg <robert@sixbynine.org>
Signed-off-by: Mika Kuoppala <mika.kuoppala@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170327203236.18276-1-matthew.auld@intel.com