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

drivers/gpu/drm/i915/i915_params.c

@@ -224,6 +224,6 @@ module_param_named(enable_dpcd_backlight, i915.enable_dpcd_backlight, bool, 0600
 MODULE_PARM_DESC(enable_dpcd_backlight,
 	"Enable support for DPCD backlight control (default:false)");
 
-module_param_named(enable_gvt, i915.enable_gvt, bool, 0600);
+module_param_named(enable_gvt, i915.enable_gvt, bool, 0400);
 MODULE_PARM_DESC(enable_gvt,
 	"Enable support for Intel GVT-g graphics virtualization host support(default:false)");