drm: mali-dp: fix stride setting for multi-plane formats

Hardware has multiple (2 or 3, depending on model) stride
registers per layer; add a function that correctly takes that
into account. On hardware that only has 2 stride registers,
ensure that 3-plane (YUV) content has identical strides
for both chroma planes.

Signed-off-by: Mihail Atanassov <mihail.atanassov@arm.com>
[Removed smart layer stride setup, comment and commit message clarifications]
Signed-off-by: Liviu Dudau <Liviu.Dudau@arm.com>