drm/vmwgfx: Fix mob cursor allocation race

Writes to SVGA_REG_CURSOR_MOBID did not wait for the buffers to be fully
populated. This sometimes results in the device not being aware of
the buffer when the cursor mob register was written.
Properly wait for the buffer to be fully populated before setting it
as a cursor mob.

Signed-off-by: Zack Rusin <zackr@vmware.com>
Fixes: 485d98d4 ("drm/vmwgfx: Add support for CursorMob and CursorBypass 4")
Reviewed-by: Martin Krastev <krastevm@vmware.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220318174332.440068-3-zack@kde.org