drm/i915/gsc: add initial support for GSC proxy

The GSC uC needs to communicate with the CSME to perform certain
operations. Since the GSC can't perform this communication directly
on platforms where it is integrated in GT, i915 needs to transfer the
messages from GSC to CSME and back.
The proxy flow is as follow:
1 - i915 submits a request to GSC asking for the message to CSME
2 - GSC replies with the proxy header + payload for CSME
3 - i915 sends the reply from GSC as-is to CSME via the mei proxy
    component
4 - CSME replies with the proxy header + payload for GSC
5 - i915 submits a request to GSC with the reply from CSME
6 - GSC replies either with a new header + payload (same as step 2,
    so we restart from there) or with an end message.

After GSC load, i915 is expected to start the first proxy message chain,
while all subsequent ones will be triggered by the GSC via interrupt.

To communicate with the CSME, we use a dedicated mei component, which
means that we need to wait for it to bind before we can initialize the
proxies. This usually happens quite fast, but given that there is a
chance that we'll have to wait a few seconds the GSC work has been moved
to a dedicated WQ to not stall other processes.

v2: fix code style, includes and variable naming (Alan)
v3: add extra check for proxy status, fix includes and comments

Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Alan Previn <alan.previn.teres.alexis@intel.com>
Reviewed-by: Alan Previn <alan.previn.teres.alexis@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230502163854.317653-4-daniele.ceraolospurio@intel.com