drm/intel: Fix initialization if startup happens in interlaced mode [v2]

My EFI BIOS starts the graphics card up in my projector's preferred EDID
mode, 1080@60i. The Intel driver does not clear all the interlaced bits.

This patch introduces a new PIPECONF_INTERLACE_MASK define and uses it
to restore progressive mode.

Signed-of-by: Christian Schmidt <schmidt@digadd.de>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Dave Airlie <airlied@redhat.com>