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

This creates read_gp/write_gp, read_dc/write_dc, read_vp/write_vp, and
read_fp/write_fp for reading and updating those registers.  Note that we don't
follow the 'DF' naming; those will be renamed to VP shortly.

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