drm/amdgpu: set vm size and block size by individual gmc by default (v3)

By default, the value is set by individual gmc.
if a specific value is input, it overrides the global value for all

v2: create helper funcs
v3: update gmc9 APU's num_level athough it may be updated in the future.

Signed-off-by: Junwei Zhang <Jerry.Zhang@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>