drm/i915/dsb: Provide intel_dsb_head() and intel_dsb_size()

The upcoming flip queue implementation will need to know the
DSB buffer head and size. Expose those outside intel_dsb.c.

Reviewed-by: Uma Shankar <uma.shankar@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250612145018.8735-3-ville.syrjala@linux.intel.com