Currently we only write as many dwords into the hardware
ELD buffers as drm_eld_size() tells us. That could mean the
remainder of the hardware buffer is left with whatever
stale garbage it had before, which doesn't seem entirely
great. Let's zero out the remainder of the buffer in case
the provided ELD doesn't fill it fully.

We can also sanity check out idea of the hardware ELD buffer's
size by making sure the address wrapped back to zero once
we wrote the entire buffer.

Cc: Chaitanya Kumar Borah <chaitanya.kumar.borah@intel.com>
Cc: Kai Vehmanen <kai.vehmanen@linux.intel.com>
Cc: Takashi Iwai <tiwai@suse.de>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Kai Vehmanen <kai.vehmanen@linux.intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221026170150.2654-11-ville.syrjala@linux.intel.com