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

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: 1629224324b6 ("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