drm/i915/guc/slpc: Add debugfs for SLPC info

This prints out relevant SLPC info from the SLPC shared structure.

We will send a H2G message which forces SLPC to update the
shared data structure with latest information before reading it.

v2: Address review comments (Michal W)
v3: Remove unnecessary tasks from slpc_info (Michal W)
v4: Rename function to intel_guc_slpc_print_info() (Michal W)
v5: checkpatch()

Reviewed-by: Michal Wajdeczko <michal.wajdeczko@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/20210730202119.23810-10-vinay.belgaumkar@intel.com