Hidden cursors also have their image updated; re-enabling the cursor
each time the image is set will cause it to re-appear.

 * Unifies the code that was in  drmmode_load_cursor_argb and
  drm_mode_show_cursor and moves it to a new drmmode_set_cursor

 * Add a new boolean, 'cursor_up', to the per-crtc
   private data to track whether the cursor should be displayed.

 * Call drmmode_set_cursor from drm_mode_show_cursor and, if
   the cursor should be displayed, from drm_mode_load_cursor_argb.

v2: Call drmModeSetCursor2 when loading a new cursor image if the
    cursor should be displayed.

Signed-off-by: Keith Packard <keithp@keithp.com>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>