drm/pci: Hide legacy PCI functions from non-legacy code

Declarations of drm_legacy_pci_{init,exit}() are being moved to
drm_legacy.h. CONFIG_DRM_LEGACY protects the implementation.

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/20191203100406.9674-3-tzimmermann@suse.de