ati-vga: Simplify pointer image handling

Rewrite reading of mouse pointer image. I am not sure this is entirely
correct but appears to work at least on little endian host with PPC
guests using little or big endian frame buffer (MorphOS and MacOS) but
still produces broken pointer image with Linux where I am not sure if
it is a guest driver bug or still missing something.

Signed-off-by: BALATON Zoltan <balaton@eik.bme.hu>
Message-ID: <b9de530074b954d661a0eb9b8b4ad82a66085456.1774110169.git.balaton@eik.bme.hu>
[PMD: Replaced BIT() -> BIT_ULL() in ati_cursor_draw_line()]
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>