drm/amd/amdgpu/amdgpu_gmc: Demote one and fix another function header

Fixes the following W=1 kernel build warning(s):

 drivers/gpu/drm/amd/amdgpu/amdgpu_gmc.c:69: warning: Function parameter or member 'bo' not described in 'amdgpu_gmc_pd_addr'
 drivers/gpu/drm/amd/amdgpu/amdgpu_gmc.c:121: warning: Function parameter or member 'bo' not described in 'amdgpu_gmc_agp_addr'
 drivers/gpu/drm/amd/amdgpu/amdgpu_gmc.c:121: warning: Excess function parameter 'tbo' description in 'amdgpu_gmc_agp_addr'

Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: "Christian König" <christian.koenig@amd.com>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: amd-gfx@lists.freedesktop.org
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>