If for some unexpected reason the registers all read zero it's better
to WARN and return instead of dividing by zero and completely freezing
the machine.

See commit 0e005888 ("drm/i915: avoid division by zero on
cnl_calc_wrpll_link") for detail.
Signed-off-by: Young Xiao <YangX92@hotmail.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/SG2PR01MB2169F6E95BC8BB5E29477042ADBC0@SG2PR01MB2169.apcprd01.prod.exchangelabs.com