drm/i915: Sanitize BIOS debugging bits from PIPECONF

Quoting the BSpec from time immemorial:

  PIPEACONF, bits 28:27: Frame Start Delay (Debug)

  Used to delay the frame start signal that is sent to the display planes.
  Care must be taken to insure that there are enough lines during VBLANK
  to support this setting.

An instance of the BIOS leaving these bits set was found in the wild,
where it caused our modesetting to go all squiffy and skewiff.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=47271


Reported-and-tested-by: Eva Wang <evawang@linpus.com>
Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=43012


Reported-and-tested-by: Carl Richell <carl@system76.com>
Cc: stable@kernel.org
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>