drm/amdgpu/vce1: Don't repeat GTT MGR node allocation

drm/amdgpu/vce1: Don't repeat GTT MGR node allocation

Only allocate entries from the GTT manager when the
VCE GTT node is not allocated yet. This prevents the
possibility of allocating them multiple times, which
causes issues during GPU reset and suspend/resume.

Fixes: 71aec08f80e7 ("amdgpu/vce: use amdgpu_gtt_mgr_alloc_entries")
Signed-off-by: Timur Kristóf <timur.kristof@gmail.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>