drm/xe/gsc: Make GSC FW load optional for newer platforms

drm/xe/gsc: Make GSC FW load optional for newer platforms

On newer platforms GSC FW is only required for content protection
features, so the core driver features work perfectly fine without it
(and we did in fact not enable it to start with on PTL). Therefore, we
can selectively enable the GSC only if the FW is found on disk, without
failing if it is not found.

Note that this means that the FW can now be enabled (i.e., we're looking
for it) but not available (i.e., we haven't found it), so checks on FW
support should use the latter state to decide whether to go on or not.

As part of the rework, the message for FW not found has been cleaned up
to be more readable.

While at it, drop the comment about xe_uc_fw_init() since the code has
been reworked and the statement no longer applies.

Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Julia Filipchuk <julia.filipchuk@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Julia Filipchuk <julia.filipchuk@intel.com>
Link: https://patch.msgid.link/20260108011340.2562349-6-daniele.ceraolospurio@intel.com