Fix colors for Mach64

Use the same color-calculating algorithm as in atyfb_imageblit in this
driver or in generic cfb_fillrect.

This patch fixes bad colors when using an accelerator in 15-bit and
16-bit modes.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>