drm/i915/hti: avoid theoretically possible negative shift

If phy is PHY_NONE, the shift to register bits becomes negative. Check
and warn about this.
Reported-by: coverity-bot <keescook@chromium.org>
References: https://lore.kernel.org/r/202211180848.D39006C@keescookSigned-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20221122120948.3436180-1-jani.nikula@intel.com

drivers/gpu/drm/i915/display/intel_hti.c

@@ -21,6 +21,9 @@ void intel_hti_init(struct drm_i915_private *i915)
 
 bool intel_hti_uses_phy(struct drm_i915_private *i915, enum phy phy)
 {
+	if (drm_WARN_ON(&i915->drm, phy == PHY_NONE))
+		return false;
+
 	return i915->display.hti.state & HDPORT_ENABLED &&
 		i915->display.hti.state & HDPORT_DDI_USED(phy);
 }