drm/i915/params: fix i915.fake_lmem_start module param sysfs permissions

fake_lmem_start does not need to be mutable via module param sysfs. It's
only used during driver probe.

Fixes: 16292243 ("drm/i915/lmem: add the fake lmem region")
Cc: Matthew Auld <matthew.auld@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200601215510.18379-2-jani.nikula@intel.com

drivers/gpu/drm/i915/i915_params.c

@@ -173,7 +173,7 @@ i915_param_named(enable_gvt, bool, 0400,
 #endif
 
 #if IS_ENABLED(CONFIG_DRM_I915_UNSTABLE_FAKE_LMEM)
-i915_param_named_unsafe(fake_lmem_start, ulong, 0600,
+i915_param_named_unsafe(fake_lmem_start, ulong, 0400,
 	"Fake LMEM start offset (default: 0)");
 #endif