drm/pci: Move drm_pci_alloc/free under CONFIG_DRM_LEGACY

All other users have been removed, yay!

Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
[fix checkpatch warning]
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200403110610.2344842-1-daniel.vetter@ffwll.ch