drm/pci: Only build drm_pci.c if CONFIG_PCI is set

Non-PCI systems should not build PCI helpers. Set up source code, header
file and Makefile accordingly.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191203100406.9674-2-tzimmermann@suse.de