drm/amdgpu:cleanup GMC & gart garbage function

for gart_ram_alloc/free, they are never used in driver thus
ripe them out totally.

for gart_vram_pin/unpin, they are not needed becuase we can
use bo_creat_kernel/free to replace the original manual way
in the gart_vram_alloc/free, thus gart_vram_pin/unpin can
also be riped out.

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