drm/i915: drop write perm from module params which don't support changing

We've given write permissions to dynamically change some module
parameters through /sys/module/i915/parameters although they only
support setting on module load. Fix the permissions.

Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1455621493-6865-1-git-send-email-jani.nikula@intel.com