The hw cursor is relatively adept at triggering underflows, which
manifest as a "blue flash" (since blue is configured as the underflow
color).  Juggle a few things around to tighten up the timing for setting
cursor registers in DONE irq.

And most importantly, don't ever disable the hw cursor.  Instead flip it
to a blank/empty cursor.  This seems far more reliable, as even simply
clearing the cursor-enable bit (with no other updates in previous/
following frames) can in some cases cause underflow.

v1: original
v2: add missing locking spotted by Micah

Cc: Micah Richert <richert@braincorporation.com>
Signed-off-by: Rob Clark <robdclark@gmail.com>