drm/i915: Fix legacy cursor vs. watermarks for ILK-BDW

In order to make cursor updates actually safe wrt. watermark programming
we have to clear the legacy_cursor_update flag in the atomic state. That
will cause the regular atomic update path to do the necessary vblank
wait after the plane update if needed, otherwise the vblank wait would
be skipped and we'd feed the optimal watermarks to the hardware before
the plane update has actually happened.

To make the slow vs. fast path determination in
intel_legacy_cursor_update() a little simpler we can ignore the actual
visibility of the plane (which can only get computed once we've already
chosen out path) and instead we simply check whether the fb is being
set or cleared by the user. This means a fully clipped but logically
visible cursor will be considered visible as far as watermark
programming is concerned. We can do that for the cursor since it's a
fixed size plane and the clipped size doesn't play a role in the
watermark computation.

This should fix underruns that can occur when the cursor gets
enable/disabled or the size gets changed. Hopefully it's good enough
that only pure cursor movement and flips go through unthrottled.

Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Uwe Kleine-König <uwe@kleine-koenig.org>
Reported-by: Uwe Kleine-König <uwe@kleine-koenig.org>
Fixes: f79f2692

 ("drm/i915: Add a cursor hack to allow converting legacy page flip to atomic, v3.")
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170217150159.11683-1-ville.syrjala@linux.intel.com
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Tested-by: Rafael Ristovski <rafael.ristovski@gmail.com>