drm/vc4: Don't wait for vblank when updating the cursor

Commonly used desktop environments such as xfce4 and gnome
on debian sid can flood the graphics drivers with cursor
updates.  Because the current implementation is waiting
for a vblank between cursor updates, this will cause the
display to hang for a long time since a typical refresh
rate is only 60Hz.

This is unnecessary and unexpected by user mode software,
so simply swap out the cursor frame buffer without waiting.

Signed-off-by: Michael Zoran <mzoran@crowfest.net>
Reviewed-by: Eric Anholt <eric@anholt.net>
Link: http://patchwork.freedesktop.org/patch/msgid/20170224015431.24583-1-mzoran@crowfest.net