drm/omapdrm: drop use of drmP.h

Drop use of the deprecated header drmP.h.
Rearranged list of include files to match rest of
DRM too.
The drmP.h file was deleted from the header file, and the necessary
includes was added to the .c files to fix build.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Acked-by: Emil Velikov <emil.velikov@collabora.com>
Cc: Tomi Valkeinen <tomi.valkeinen@ti.com>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20190716064220.18157-10-sam@ravnborg.org