drm/i915: Nuke a pointless unreachable()

The unreachable() is very much unreachable and the compiler knows
that, so there's no point in having it.

Cc: Ben Widawsky <ben@bwidawsk.net>
Cc: Jason Ekstrand <jason@jlekstrand.net>
Cc: Daniel Stone <daniels@collabora.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171222192231.17981-3-ville.syrjala@linux.intel.comReviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_display.c

@@ -12950,8 +12950,6 @@ static bool intel_primary_plane_format_mod_supported(struct drm_plane *plane,
 		return i965_mod_supported(format, modifier);
 	else
 		return i8xx_mod_supported(format, modifier);
-
-	unreachable();
 }
 
 static bool intel_cursor_plane_format_mod_supported(struct drm_plane *plane,