drm/i915: Reassign DPLLs only for crtcs going throug .compute_config()

Only reassign the pipe's DPLL if it's going through a full
.compute_config() cycle. If OTOH it's just getting modeset
eg. in order to change cdclk there doesn't seem much point in
picking a new DPLL for it.

This should also prevent .get_dplls() from seeing a funky port_clock
for DP even in cases where the readout produces a non-standard
clock and we (for some reason) have decided to not fully recompute
the state to remedy the situation.

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/20220907091057.11572-7-ville.syrjala@linux.intel.com