drm/i915/guc/slpc: Enable ARAT timer interrupt

This interrupt is enabled during RPS initialization, and
now needs to be done by SLPC code. It allows ARAT timer
expiry interrupts to get forwarded to GuC.

v2: Fix comment (Matthew Brost)
v3: checkpatch()

Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Signed-off-by: Vinay Belgaumkar <vinay.belgaumkar@intel.com>
Signed-off-by: John Harrison <John.C.Harrison@Intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210730202119.23810-11-vinay.belgaumkar@intel.com