drm/{i915, xe}/dsb: move DSB buffer to parent interface

drm/{i915, xe}/dsb: move DSB buffer to parent interface

Move the DSB buffer handling to the display parent interface, making
display more independent of i915 and xe driver implementations.

Since the DSB parent interface is only called from intel_dsb.c, add the
wrappers there with smaller visibility instead of the usual
intel_parent.[ch], and using struct intel_dsb as the context parameter
for convenience.

Unfortunately, memset() being a macro in linux/fortify-string.h, we
can't use that as the function pointer name. dsb->memset() would be
using the macro and leading to build failures. Therefore, use .fill()
for the memset() functionality.

v2: s/memset/fill/

Reviewed-by: Michał Grzelak <michal.grzelak@intel.com>
Link: https://patch.msgid.link/df117c862a6d34dae340e4a85c2482b4e29c8884.1768923917.git.jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>