drm: Remove pdev field from struct drm_device

The field pdev in struct drm_device is unused. Remove it. The
value can be obtained by upcasting from the structure's dev field.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/20210502104953.21768-8-tzimmermann@suse.de