drm: use getter/setter functions

Use getter and setter functions, for platform_device structures and a
mipi_dsi_device structure.

Signed-off-by: Julia Lawall <Julia.Lawall@inria.fr>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210209211304.1261740-1-Julia.Lawall@inria.fr