drm/i915/guc/slpc: Enable SLPC and add related H2G events

Add methods for interacting with GuC for enabling SLPC. Enable
SLPC after GuC submission has been established. GuC load will
fail if SLPC cannot be successfully initialized. Add various
helper methods to set/unset the parameters for SLPC. They can
be set using H2G calls or directly setting bits in the shared
data structure.

v2: Address several review comments, add new helpers for
decoding the SLPC min/max frequencies. Use masks instead of hardcoded
constants. (Michal W)

v3: Split global_state_to_string function, and check for positive
non-zero return value from intel_guc_send() (Michal W)

v4: Optimize the stringify function and other comments (Michal W)

v5: Enable slpc as well before declaring GuC submission status (Michal W)

v6: 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/20210730202119.23810-6-vinay.belgaumkar@intel.com