drm/i915/dsi: Don't set/read the DSI C clock divider on GLK

drm/i915/dsi: Don't set/read the DSI C clock divider on GLK

GLK doesn't use the DSI C clock at all, no need to program
the divider for it. Bspec even says: "Do not program this field".

However looks like some firmware versions program this and
some do not. In order to avoid bogus fastset mismatches
we should also filter it out during readout.

v2: Clear all the DSI C clock bits during readout (Jani)
    Adjust platform checks for new style, and add
    has_dsic_clock() while at it.

Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250718112928.27669-1-ville.syrjala@linux.intel.com