drm/i915: Remove modeset lock check from intel_pipe_update_start()

A follow up patch will call this funcion from a work context for the
mmio flip, in which case we cannot acquire the modeset locks. That's
not a problem though, since the check is there to protect vblank and
the mode, but the code that changes that waits for pending flips
first.

Signed-off-by: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>