drm/i915/dsb: Polish the DSB ID enum

drm/i915/dsb: Polish the DSB ID enum

Namespace the DSB ID enum properly, and make the naming match
other such enums in general. Also make the names 0 based as
that's what Bspec uses for DSB (unlike eg. planes where it
uses 1 based indexing).

We'll throw out INVALID_DSB while at it since we have no use for
it at the moment.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240531114101.19994-2-ville.syrjala@linux.intel.com
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>