drm: Remove the struct drm_device platformdev field

The field contains a pointer to the parent platform device of the DRM
device. As struct drm_device also contains a dev pointer to the struct
device embedded in the platform_device structure, the platformdev field
is redundant. Remove it and use the dev pointer directly.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: Jyri Sarha <jsarha@ti.com>
Acked-by: Vincent Abriou <vincent.abriou@st.com> # For sti
Acked-by: Russell King <rmk+kernel@armlinux.org.uk> # For armada
Acked-by: Rob Clark <robdclark@gmail.com> # For msm
Acked-by: Xinwei <Kong&lt;kong.kongxinwei@hisilicon.com>