drm: mali-dp: Separate static internal data into a read-only structure.

The malidp_hw_device structure that the driver uses to handle the
differences between versions of the IP contains both non-changeable
data and fields that get updated at probe time. Previously we were
copying the read-only part into allocated memory, but that can be
completely avoided by splitting the structure into a read-only part
and keeping the runtime modifiable fields into the old structure.

Reviewed-by: Brian Starkey <brian.starkey@arm.com>
Reviewed-by: Gustavo Padovan <gustavo.padovan@collabora.com>
Signed-off-by: Liviu Dudau <liviu.dudau@arm.com>