st/nine: Don't call SetCursor until a cursor is set

st/nine: Don't call SetCursor until a cursor is set

The previous code was ignoring the input
until a cursor is set inside d3d
(with SetCursorProperties), as expected
by wine tests.

However it did still make a call to ID3DPresent_SetCursor,
which would result into a SetCursor(NULL) call, thus
hidding any cursor set outside d3d, which we shouldn't do.

Add comment about not avoiding redundant ID3DPresent_SetCursor
calls once a cursor has been set in d3d, as it has been tested to
cause regressions.

Fixes: https://github.com/iXit/Mesa-3D/issues/197
Signed-off-by: Axel Davy <davyaxel0@gmail.com>