drm: Upcast struct drm_device.dev to struct pci_device; replace pdev

We have DRM drivers based on USB, SPI and platform devices. All of them
are fine with storing their device reference in struct drm_device.dev.
PCI devices should be no exception. Therefore struct drm_device.pdev is
deprecated.

Instead upcast from struct drm_device.dev with to_pci_dev(). PCI-specific
code can use dev_is_pci() to test for a PCI device. This patch changes
the DRM core code and documentation accordingly.

v4:
	* split-off pdev deprecation into separate patch

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Tested-by: Andy Lavr <andy.lavr@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210118131420.15874-2-tzimmermann@suse.de