drm/i915: Convert DG1 over to .{enable,disable}_clock()

Replace dg1_{map,unmap}_plls_to_ports() with the appropriate
encoder vfuncs. And let's relocate the disable function next to
the enable function while at it.

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/20210205214634.19341-7-ville.syrjala@linux.intel.com