drm/amdgpu: add df perfmon regs and funcs for xgmi

v6: Squash in warning fix (Colin Ian King)
v5: Fix warnings (Alex)
v4: fixed mixed delaration and code warnings and minor errors
v3: exposing df funcs in amdgpu_df_funcs in amdgpu.h
v2: moving permonctl/perfmonctr from default to offset

- adding df perfmonctl and perfmonctr registers for df counters
- adding df funcs to set perfmonctl and get perfmonctr for
df and xgmi counters
- exposing df funcs in amdgpu_df_funcs

Signed-off-by: Jonathan Kim <jonathan.kim@amd.com>
Reviewed-by: Evan Quan <evan.quan@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>