drm/panthor: Ensure MCU is disabled on suspend

Currently the Panthor driver needs the GPU to be powered down
between suspend and resume. If this is not done, then the
MCU_CONTROL register will be preserved as AUTO, which again will
cause a premature FW boot on resume. The FW will go directly into
fatal state in this case.

This case needs to be handled as there is no guarantee that the
GPU will be powered down after the suspend callback on all platforms.

The fix is to call panthor_fw_stop() in "pre-reset" path to ensure
the MCU_CONTROL register is cleared (set DISABLE). This matches
well with the already existing call to panthor_fw_start() from the
"post-reset" path.

Signed-off-by: Ketil Johnsen <ketil.johnsen@arm.com>
Acked-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Steven Price <steven.price@arm.com>
Fixes: 2718d918 ("drm/panthor: Add the FW logical block")
Signed-off-by: Steven Price <steven.price@arm.com>
Link: https://lore.kernel.org/r/20251008105112.4077015-1-ketil.johnsen@arm.com