drm/tyr: rename pad0 to selected_coherency

drm/tyr: rename pad0 to selected_coherency

This applies the uapi change in commit ea78ec982653 ("drm/panthor:
Expose the selected coherency protocol to the UMD") to the Tyr driver as
well.

Once this is merged with drm-rust-next, this kind of change to the uapi
struct is automatically reflected in the Tyr driver's GpuInfo. [1]

Link: https://lore.kernel.org/r/aW8-oH7dtp-OTAZC@google.com
Acked-by: Danilo Krummrich <dakr@kernel.org>
Link: https://patch.msgid.link/20260120-tyr-pad0-coherency-v1-1-91f40e56c67a@google.com
Signed-off-by: Alice Ryhl <aliceryhl@google.com>