drm/dp: drmP.h include removal

Continue to get rid of drmP.h. Add minimal includes to build. Sort
includes while at it.

Reviewed-by: Noralf Trønnes <noralf@tronnes.org>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190506095248.20874-1-jani.nikula@intel.com