drm/i915: Don't look at unrelated PIPECONF bits for interlaced readout

drm/i915: Don't look at unrelated PIPECONF bits for interlaced readout

Since HSW the PIPECONF progressive vs. interlaced selection is done
with just two bits instead of the earlier three. Let's not look at the
extra bit on HSW+. Also gen2 doesn't support interlaced displays at all.

This is actually fine as is currently because the extra bit is mbz (as
are all three bits on gen2). But just to avoid mishaps in the future
if the bits get reused let's only look at what's properly defined.

v2: constify crtc_state

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190718145053.25808-8-ville.syrjala@linux.intel.com
Reviewed-by: Gwan-gyeong Mun <gwan-gyeong.mun@intel.com>