drm/i915: Introduce .{enable,disable}_clock() encoder vfuncs

The current code dealing with the clock routing for DDI encoders
is a maintenance nightmare. Let's start cleaning it up by allowing
the encoder to provide vfuncs for enablign/disabling the clock.

We leave them initially unimplemented, falling back to the old
if-else approach.

v2: Convert the FDI enable sequence

Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com> #v2
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210205214634.19341-3-ville.syrjala@linux.intel.com