drm/amdgpu: Allocate GART table in RAM for AMD APU

Some AMD APUs may not have a dedicated VRAM. On such platforms the GART
table should be allocated on the system memory. When real vram size is
zero, place the GART table in system memory and create an SG BO to make
it GPU accessible.

v2: fix includes

Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
(rajneesh: removed set_memory_wc workaround)
Signed-off-by: Rajneesh Bhardwaj <rajneesh.bhardwaj@amd.com>
Signed-off-by: Harish Kasiviswanathan <Harish.Kasiviswanathan@amd.com>
Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>