drm/i915: Don't sanitize frame start delay if the pipe is off

Avoid touching PIPECONF in intel_sanitize_crtc() unless the pipe is
actually on. Should cure some unclaimed register accesses during reset,
as we are rather cavalier in our approach to powerdomain management.

We don't have to sanitize this if the pipe is off since we will
overwrite the frame start delay anyway when turning the pipe on.

v2: Amended commit message to implicate the reset path (Chris)

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=102249
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171115200442.15051-1-ville.syrjala@linux.intel.com
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>