drm/selftests: drop use of drmP.h

Drop the single user of drmP.h - replace it with relevant includes.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Acked-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Alexandru-Cosmin Gheorghe <Alexandru-Cosmin.Gheorghe@arm.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190716064220.18157-11-sam@ravnborg.org