drm/i915/dsb: DSB code refactoring

Refactor DSB implementation to be compatible with Xe driver.

v1: RFC version.
v2: Make intel_dsb structure opaque from external usage. [Jani]
v3: Rebased on latest.
v4:
- Add boundary check in dsb_buffer_memset(). [Luca]
- Use size_t instead of u32. [Luca]
v5: WARN_ON() added for out of boudary case with some optimization. [Luca]
v6: Rebased on latest and fix a rebase-miss.

Cc: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Luca Coelho <luciano.coelho@intel.com>
Signed-off-by: Animesh Manna <animesh.manna@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231110032518.3564279-1-animesh.manna@intel.com