drm: Remove drmP.h include from drm_kms_helper_common.c

An easy one as a drive-by.

Reviewed-by: Gustavo Padovan <gustavo.padovan@collabora.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170308141257.12119-6-daniel.vetter@ffwll.ch