drm/nouveau: make cursor_set implementation consistent with other drivers

When xorg state tracker wants to hide the cursor it calls set_cursor
with NULL buffer_handle and size=0x0, but nouveau refuses to hide it
because size is not 64x64... which is a bit odd. Both radeon and intel
check buffer_handle before validating size of cursor, so make nouveau
implementation consistent with them.

Signed-off-by: Marcin Slusarz <marcin.slusarz@gmail.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>