drm/i915/dsb: Move DSB ID definition to the header

We're going to need to make the DSB ID visible outside the DSB
code, so that we eg. can use multiple DSB engines in parallel.
to that end move the definition to intel_dsb.h.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240531114101.19994-3-ville.syrjala@linux.intel.com


Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>