drm/i915: Sprinkle WARN(!pll) into icl/dg1 .clock_enable()

The other DDI .enable_clock() functions are trying to protect us
against pll==NULL. A bit tempted to throw out all the WARNs as
just unnecessary noise, but I guess they might have some use
when poking around the shared_dpll code (not sure it wouldn't
oops elsewhere though). So let's unify it all and sprinkle in
the missing WARNs for icl/dg1.

Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210205214634.19341-11-ville.syrjala@linux.intel.com