drm/i915/guc/slpc: Add SLPC selftest

drm/i915/guc/slpc: Add SLPC selftest

Tests that exercise the SLPC get/set frequency interfaces.

Clamp_max will set max frequency to multiple levels and check
that SLPC requests frequency lower than or equal to it.

Clamp_min will set min frequency to different levels and check
if SLPC requests are higher or equal to those levels.

v2: Address review comments (Michal W)
v3: Checkpatch() corrections
v4: Remove unnecessary header file (Matthew Brost)
v5: checkpatch() and define const for 50/3 (Matthew Brost)

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/20210730202119.23810-14-vinay.belgaumkar@intel.com