drm/sysfs: Expose DRM connector id in each connector sysfs

Expose DRM connector id in device sysfs so that we can map the connector
id to the connector syspath. Currently, even if we can derive the
connector id from modeset, we do not have a way to find the
corresponding connector's syspath.

This is helpful when determining the root connector of MST tree. When a
tree of multiple MST hub is connected to the system, modeset describes
the tree in the PATH blob. For example, consider the following scenario.

+-------------+
| Source      |    +-------------+
| (Device)    |    | BranchX     |
|             |    | (MST)       |
|       [conn6]--->|       [port1]--->DisplayA
+-------------+    |             |
                   |             |    +-------------+
                   |             |    | BranchY     |
                   |             |    | (MST)       |
                   |       [port2]--->|       [port1]----->DisplayB
                   +-------------+    |             |
                                      |       [port2]----->DisplayC
                                      +-------------+

DPMST connector of DisplayA would have "mst:6-1" PATH.
DPMST connector of DisplayB would have "mst:6-2-1" PATH.
DPMST connector of DisplayC would have "mst:6-2-2" PATH.

Given that connector id of 6 is the root of the MST connector tree, we
can utilize this patch to parse through DRM connectors sysfs and find
which connector syspath corresponds to the root connector (id == 6).

ChromeOS intend to use this information for metrics collection. For
example, we want to tell which port is deriving which displays even with
a MST hub. Chromium patch for parsing DRM connector id from the kernel
is at http://crrev.com/c/4317207.

Signed-off-by: Won Chung <[email protected]>
Reviewed-by: Manasi Navare <[email protected]>
Signed-off-by: Manasi Navare <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

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