gxfb: clean up register definitions

This does the following in preparation for register saving:
  - moves the register definitions from video_gx.h and display_gx.h into
    gxfb.h.
  - renames GX_* registers to match their section (ie, VP_).
  - renames register bitfields to match the data sheet (ie,
    DC_DCFG_TGEN -> DC_DISPLAY_CFG_TGEN).
  - for DC registers, rather than defining to specific addresses, use
    an enum to number them sequentially and just multiply by 4(bytes) to
    access them (in read_dc/write_dc).
  - for VP and FP registers, use an enum and multiple by 8 (bytes).  They're
    64bit registers.

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>