drm/i915/dsb: Introduce intel_dsb_poll()

Add a function for emitting a DSB poll instruction. We'll allow
the caller to specify the poll parameters.

v2: s/wait/wait_us/ (Ankit)

Reviewed-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250207223159.14132-6-ville.syrjala@linux.intel.com