drm/i915: Use standard form -EDEADLK check

Unify how we check for -EDEADLK vs. other errors from
crtc vs. encoder compute_config() calls.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210930104133.30854-1-ville.syrjala@linux.intel.com
Reviewed-by: Jani Nikula <jani.nikula@intel.com>