drm/i915: Put the downclock_mode check back into can_enable_drrs()

drm/i915: Put the downclock_mode check back into can_enable_drrs()

With static DRRS the user might ask for the lowest possible refresh
rate of the panel, in which case we're not going to find a suitable
downclock mode for it and we should not try to enable seamless DRRS.
This will in fact oops.

We used to check for the presence of the downclock mode here, but
that got removed in commit 5bbc5afa3eba ("drm/i915: Introduce
intel_panel_drrs_type()") as redundant (which it was at the time).
But we do need the check again now that static DRRS is a thing.

I must have not re-tested static DRRS fully after introducing
intel_panel_drrs_type() :/

Fixes: 6895ebb6e300 ("drm/i915: Implement static DRRS")
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220315132752.11849-2-ville.syrjala@linux.intel.com
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>