xf86drm: introduce a get_real_pci_path() helper

Introduce a helper which gets the real sysfs path for the given pci
device.

In other words, instead opening the /sys/dev/char/*/device symlink, we
opt for the actual /sys/devices/pci*/*/

It folds three (nearly identical) snprintf's and paves the way of adding
extra devices (see next patch) a piece of pie.

v2: use a caller (on stack) provided real_path (Eric)

Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Tested-by: Robert Foss <robert.foss@collabora.com> (v1)
Reviewed-by: Robert Foss <robert.foss@collabora.com> (v1)
Reviewed-by: Eric Engestrom <eric@engestrom.ch> (v1)