drm/amdgpu: move more logic into amdgpu_vm_map_gart v3

No need to duplicate that code over and over again. Also stop using the
flags to determine if we need to map the addresses.

v2: constify the pages_addr
v3: rebased, fix typo in commit message

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