drm/amdgpu: use idr_init_base() to initialize mgr->ctx_handles

idr_init_base(), implemented by commit 6ce711f2 ("idr: Make 1-based
IDRs more efficient"), let us set an arbitrary base other than
idr_init(), which uses base 0.

Since, for this IDR, no ID < 1 is ever requested, using
idr_init_base(&idr, 1) avoids unnecessary tree walks.

Signed-off-by: Danilo Krummrich <dakr@redhat.com>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://lore.kernel.org/r/20220701185303.284082-2-dakr@redhat.com