omap: include omap_drm.h independently

omap_drm.h uses data type defined in stdint.h, but that header was
not included.

omap_drm.h includes drm.h as a local file when it is part of the
compiler c flags.

This two issues are fixed. New code can include omap_drm.h alone.

Signed-off-by: Víctor Manuel Jáquez Leal <vjaquez@igalia.com>
Signed-off-by: Rob Clark <rob@ti.com>