drm/pci: add missing variable initialization

Fixed build warning as below:

drivers/gpu/drm/drm_pci.c: In function 'drm_pcie_get_speed_cap_mask':
drivers/gpu/drm/drm_pci.c:496:9: warning: 'lnkcap' may be used uninitialized in this function [-Wuninitialized]
drivers/gpu/drm/drm_pci.c:497:10: warning: 'lnkcap2' may be used uninitialized in this function [-Wuninitialized]

Signed-off-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>