amdgpu/vce: use amdgpu_gtt_mgr_alloc_entries

amdgpu/vce: use amdgpu_gtt_mgr_alloc_entries

Instead of reserving a number of GTT pages for VCE 1.0 this
commit now uses amdgpu_gtt_mgr_alloc_entries to allocate
the pages when initializing vce 1.0.

While at it remove the "does the VCPU BO already have a
32-bit address" check as suggested by Timur.

This decouples vce init from gtt init.

---
v7: renamed variables (Christian)
---

Signed-off-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>