drm/i915: Allow powersave modparam to be adjusted at runtime.

2.6.36 appears to respect the 0400 mode we assigned to the parameter
preventing it from being adjusted after loading. However, this is safe
to adjust at runtime.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=31311
Reported-by: Fernando Lemos <fernandotcl@gmail.com>
Cc: stable@kernel.org
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>