drm/amd/amdgpu: port gfx6 over to new si headers (v2)

This changes the gfx v6 driver to use the new AMDGPU style
SI headers.  Also fixes a variety of coding style issues, white
space issues, and uses WREG32_FIELD in a few places where
appropriate.

Tested with a Tahiti 0x679A.

v2: Squash in typo fix patch

Signed-off-by: Tom St Denis <tom.stdenis@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>