drm/nouveau: Remove invalid reference to struct drm_device.pdev

The pdev field got removed from struct drm_device recently. Replace
the invalid reference with an upcast from the struct's dev field.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Dave Airlie <airlied@redhat.com>
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Fixes: b347e044 ("drm: Remove pdev field from struct drm_device")
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Maxime Ripard <maxime@cerno.tech>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maxime Ripard <mripard@kernel.org>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: dri-devel@lists.freedesktop.org
Link: https://patchwork.freedesktop.org/patch/msgid/20210512185527.26050-1-tzimmermann@suse.de