drm/amdkfd: simplify APU VRAM handling

With commit 89773b85
("drm/amdkfd: Let VRAM allocations go to GTT domain on small APUs")
big and small APU "VRAM" handling in KFD was unified.  Since AMD_IS_APU
is set for both big and small APUs, we can simplify the checks in
the code.

v2: clean up a few more places (Lang)

Acked-by: Felix Kuehling <felix.kuehling@amd.com>
Reviewed-by: Lang Yu <Lang.Yu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>