drm/amdgpu: add amdgpu_gmc_agp_location v3

Helper to figure out the location of the AGP BAR.

v2: fix a couple of bugs
v3: correctly add one to vram_end

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