drm: avoid includes in drm_crtc_helper_internal.h

Prefer forward declarations over includes where possible.

Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/fb7fb2986e19f949bf3a9b16230a59e0f7aaebdf.1709898638.git.jani.nikula@intel.com