drm: Read PCIER_LINK_CAP/PCIER_LINK_CAP2 from the PCI bridge

Before this fix, capabilities were read from vgapci and were incorrect.
Add missing bits from the vendor's 2005-05-04 change to
contrib/byacc/makefile.in ("add YYPATCH here so it can be tested by
applications") so that applications have a hope of detecting newer
FreeBSD YACC output from an older one.

Submitted by: Juniper Networks