drm/amdkfd: use GTT for VRAM on APUs only if GTT is larger

If the user has configured a large carveout on a small APU,
only use GTT for VRAM allocations if GTT is larger than
VRAM.

v2: fix reversed check (Philip)

Reviewed-by: Mario Limonciello <mario.limonciello@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>