drm/panic: Add missing static inline to drm_panic_is_enabled()

This breaks build if DRM_PANIC is not enabled.
Also add the missing include drm_crtc_internal.h in drm_panic.c

Fixes: 9f774c42 ("drm/panic: Add drm_panic_is_enabled()")
Signed-off-by: Jocelyn Falempe <jfalempe@redhat.com>
Reviewed-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240719122051.1507927-1-jfalempe@redhat.com