drm/i915/dsb: Introduce intel_dsb_wait_usec()

Add a function to emit the DSB "wait usecs" instruction.
This is just a usleep() for the DSB.

As a lower bound it seems pretty accurate, but the upper bound
seemed oddly relaxed (ie. sometimes I've seen waits that are
quite a bit longer than specified, not sure why).

Reviewed-by: Animesh Manna <animesh.manna@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240930170415.23841-7-ville.syrjala@linux.intel.com