drm/amd: Fix set but not used warnings

drm/amd: Fix set but not used warnings

There are many set but not used warnings under drivers/gpu/drm/amd when
compiling with the latest upstream mainline GCC:

  drivers/gpu/drm/amd/amdgpu/amdgpu_gart.c:305:18: warning: variable ‘p’ set but not used [-Wunused-but-set-variable=]
  drivers/gpu/drm/amd/amdgpu/amdgpu_vcn.h:103:26: warning: variable ‘internal_reg_offset’ set but not used [-Wunused-but-set-variable=]
  ...
  drivers/gpu/drm/amd/amdgpu/amdgpu_vcn.h:164:26: warning: variable ‘internal_reg_offset’ set but not used [-Wunused-but-set-variable=]
  ...
  drivers/gpu/drm/amd/amdgpu/../display/dc/dc_dmub_srv.c:445:13: warning: variable ‘pipe_idx’ set but not used [-Wunused-but-set-variable=]
  drivers/gpu/drm/amd/amdgpu/../display/dc/dc_dmub_srv.c:875:21: warning: variable ‘pipe_idx’ set but not used [-Wunused-but-set-variable=]

Remove the variables actually not used or add __maybe_unused attribute for
the variables actually used to fix them, compile tested only.

Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>