drm/i915: Make semaphore modparam RO

A couple patches in the upcoming rework of semaphores will break if
semaphores are toggled by the user at various times. Since the code
cleanups there seem to be an overall win, and toggling semaphores at
runtime is not a terribly useful thing to do, simply make the module
parameter read-only.

Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>