drm/lima: Clean up redundant pdev pointer

There's no point explicitly tracking the platform device when it can be
trivially derived from the regular device pointer in the couple of
places it's ever used.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Qiang Yu <yuq825@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/8d9073cc91c10fc70910587fd1794e0e8f32b467.1587509150.git.robin.murphy@arm.com