drm/xe/uc: Add GuC/HuC firmware path overrides

When testing a new binary and/or debugging binary-related issues, it is
useful to have the option to change which binary is loaded without
having to update and re-compile the kernel. To support this option, this
patch adds 2 new modparams to override the FW path for GuC and HuC. The
HuC modparam can also be set to an empty string to disable HuC loading.

Note that those modparams only take effect on platforms where we already
have a default FW, so we're sure there is support for FW loading and the
kernel isn't going to explode in an undefined path.

v2: simplify comment (John),
    rebase on s/guc_submission_enabled/uc_enabled

Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: John Harrison <John.C.Harrison@Intel.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: John Harrison <John.C.Harrison@Intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>