drm/i915/fbc: fix CFB size calculation for gen8+

Broadwell and newer actually compress up to 2560 lines instead of 2048
(as documented in the FBC_CTL page). If we don't take this into
consideration we end up reserving too little stolen memory for the
CFB, so we may allocate something else (such as a ring) right after
what we reserved, and the hardware will overwrite it with the contents
of the CFB when FBC is active, causing GPU hangs. Another possibility
is that the CFB may be allocated at the very end of the available
space, so the CFB will overlap the reserved stolen area, leading to
FIFO underruns.

This bug has always been a problem on BDW (the only affected platform
where FBC is enabled by default), but it's much easier to reproduce
since the following commit:
    commit c58b735fc762e891481e92af7124b85cb0a51fce
    Author: Chris Wilson <chris@chris-wilson.co.uk>
    Date:   Thu Aug 18 17:16:57 2016 +0100
        drm/i915: Allocate rings from stolen

Of course, you can only reproduce the bug if your screen is taller
than 2048 lines.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=98213
Fixes: a98ee79317b4 ("drm/i915/fbc: enable FBC by default on HSW and BDW")
Cc: <stable@vger.kernel.org> # v4.6+
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1477065346-13736-1-git-send-email-paulo.r.zanoni@intel.com
(cherry picked from commit 79f2624b1b9f776b173b41d743fb3dd7374b3827)
Signed-off-by: Jani Nikula <jani.nikula@intel.com>