drm/savage: drop use of drmP.h

Drop use of the deprecated drmP.h header file.
Replace it with the necessary includes of other headers.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Daniel Vetter <daniel@ffwll.ch>
Cc: David Airlie <airlied@linux.ie>
Link: https://patchwork.freedesktop.org/patch/msgid/20190605140313.13629-3-sam@ravnborg.org