drm/i915/display: remove debug message from error path

First check in the function is if swsci() is supported. All the error
paths are easy to figure out the reason, so remove the extra debug
message: it's normal not to support swsci() e.g. in dgfx.

v2: Rather than special case dgfx, just remove the debug message
    (from Ville)

Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201027044618.719064-2-lucas.demarchi@intel.com