We were reserving fewer dwords in the ring than necessary. Indeed
we're always writing all registers once, so discard the actual number
of registers given by the user and just program the whitelisted ones
once.

Fixes: 19f81df2 ("drm/i915/perf: Add OA unit support for Gen 8+")
Reported-by: Matthew Auld <matthew.william.auld@gmail.com>
Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Cc: <stable@vger.kernel.org> # v4.12+
Link: https://patchwork.freedesktop.org/patch/msgid/20170803165812.2373-6-lionel.g.landwerlin@intel.com