gxfb: set the right registers to tweak the sync polarity

While running in flatpanel mode it is important to change the FP sync bits (VG
register 0x408) rather then the CRT sync bits (VG register 0x008).  This patch
keeps the CRT sync bits at default when a flatpanel exists.

Note that this also fixes inverted logic; we want CRT_VSYNC_POL to be set (ie,
vsync is normally high) when FB_SYNC_VERT_HIGH_ACT is unset.

Signed-off-by: Jordan Crouse <jordan.crouse@amd.com>
Signed-off-by: Andres Salomon <dilinger@debian.org>
Cc: "Antonino A. Daplas" <adaplas@pol.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>