drm/i915/gen9: Store plane minimum blocks in CRTC wm state (v2)

This will eventually allow us to re-use old values without
re-calculating them for unchanged planes (which also helps us avoid
re-grabbing extra plane states).

v2:
 -  Drop unnecessary memset's; they were meant for a later patch (which
    got reworked anyway to not need them, but were mis-rebased into this
    one.  (Maarten)

Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1463061971-19638-6-git-send-email-matthew.d.roper@intel.com