drm/i915/guc/slpc: Gate Host RPS when SLPC is enabled

Also ensure uc_init is called before we initialize RPS so that we
can check for SLPC support. We do not need to enable up/down
interrupts when SLPC is enabled. However, we still need the ARAT
interrupt, which will be enabled separately later.

v2: Explicitly return from intel_rps_enable with slpc check (Matthew B)

Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Signed-off-by: Vinay Belgaumkar <vinay.belgaumkar@intel.com>
Signed-off-by: Sundaresan Sujaritha <sujaritha.sundaresan@intel.com>
Signed-off-by: John Harrison <John.C.Harrison@Intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210730202119.23810-3-vinay.belgaumkar@intel.com