drm/panthor: Explicitly set the coherency mode

This commit fixes the potential misalignment between the value of device
tree property "dma-coherent" and default value of COHERENCY_ENABLE
register.
Panthor driver didn't explicitly program the COHERENCY_ENABLE register
with the desired coherency mode. The default value of COHERENCY_ENABLE
register is implementation defined, so it may not be always aligned with
the "dma-coherent" property value.
The commit also checks the COHERENCY_FEATURES register to confirm that
the coherency protocol is actually supported or not.

v2:
- Added R-b tags

Signed-off-by: Akash Goel <akash.goel@arm.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Liviu Dudau <liviu.dudau@arm.com>
Reviewed-by: Steven Price <steven.price@arm.com>
Link: https://lore.kernel.org/r/20241030225407.4077513-3-akash.goel@arm.com


Signed-off-by: Liviu Dudau <liviu.dudau@arm.com>