drm/i915: Set the access right of kernel param "i915.enable_gvt" to read-only.

The access right of kernel param "i915.enable_gvt" should be read-only as
it only applies during module load (and is not *runtime* writable).

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Zhi Wang <zhi.a.wang@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/1466425022-3709-1-git-send-email-zhi.a.wang@intel.com