drm/amdgpu: Add amdgpu_bo_is_vm_bo helper

Help code readability by replacing a bunch of:

bo->tbo.base.resv == vm->root.bo->tbo.base.resv

With:

amdgpu_vm_is_bo_always_valid(vm, bo)

No functional changes.

v2:
 * Rename helper and move to amdgpu_vm. (Christian)

v3:
 * Use Christian's kerneldoc.

v4:
 * Fixed logic inversion in amdgpu_vm_bo_get_memory.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@igalia.com>
Cc: Christian König <christian.koenig@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>