gxfb: create DC/VP/FP-specific handlers rather than using readl/writel

This creates read_dc/write_dc, read_vp/write_vp, and read_fp/write_fp for
reading and updating those registers.  It creates gxfb.h to house these.

We also drop a no-op readl() from gx_set_mode.  Other than that, there should
be no functionality change.

Signed-off-by: Andres Salomon <dilinger@debian.org>
Cc: Jordan Crouse <jordan.crouse@amd.com>
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>