drm/tilcdc: drop use of drmP.h

Dropped drmP.h and all other header files not used by tilcdc_drv.h.
Added the minimal includes and forwards to make the header file
self-contained.

Then dropped the remaining uses of drmP.h and fixed all fall-out.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Acked-by: Emil Velikov <emil.velikov@collabora.com>
Cc: Jyri Sarha <jsarha@ti.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/20190716064220.18157-15-sam@ravnborg.org