drm/panthor: Replace cross-component register accesses with helpers

drm/panthor: Replace cross-component register accesses with helpers

Stop reaching into other components' registers directly and route those
operations through the component that owns them.

Move the timestamp/coherency helpers into panthor_gpu, add a doorbell
helper, and update call sites accordingly. This keeps register knowledge
local to each block and avoids spreading cross-component register
accesses across the driver.

This is a preparatory cleanup for using per-component iomem bases.

v3:
- Pick up Ack from Boris and R-bs from Liviu and Steve
v2:
- Fix incorrect spelling of timestamp helpers
- Fix unintended trailing backslash

Reviewed-by: Steven Price <steven.price@arm.com>
Reviewed-by: Liviu Dudau <liviu.dudau@arm.com>
Acked-by: Boris Brezillon <boris.brezillon@collabora.com>
Signed-off-by: Karunika Choo <karunika.choo@arm.com>
Tested-by: Boris Brezillon <boris.brezillon@collabora.com>
Signed-off-by: Liviu Dudau <liviu.dudau@arm.com>
Link: https://patch.msgid.link/20260427155934.416502-4-karunika.choo@arm.com