drm/pci: Unexport drm_get_pci_dev

Only user left is the shadow attach for legacy drivers.

v2: Shift the #ifdef CONFIG_DRM_LEGACY to now also include
drm_get_pci_dev() (Thomas)

Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Emil Velikov <emil.velikov@collabora.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200225165835.2394442-1-daniel.vetter@ffwll.ch