drm/i915/dsb: Try to document that DSB_STATUS bit 16 is level triggered

DSB_STATUS bit 16 is supposed to be a sticky bit informing us whether
the DSB was idle or not when the pipe's delayed vblank (when double
buffered registers latch) occurred. Unfortunately it turns out this
is a level triggred signal, ie. the bit will be set whenever the
DSB is busy during the scanline window between start of delayed
vblank and vtotal. Try to document that fact by renaming the bit.

Sadly this also thwarts my plan to use this bit to sanity check
that the (to be introduced) DSB based vblank evasion did its job
correctly. That would require an edge triggered signal instead.
So looks like we'll have to rely mostly on luck instead :(

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240611133344.30673-12-ville.syrjala@linux.intel.com
Reviewed-by: Jani Nikula <jani.nikula@intel.com>