drm/i915/guc/slpc: Add a new SLPC selftest

This test will validate we can achieve actual frequency of RP0. Pcode
grants frequencies based on what GuC is requesting. However, thermal
throttling can limit what is being granted. Add a test to request for
max, but don't fail the test if RP0 is not granted due to throttle
reasons.

Also optimize the selftest by using a common run_test function to avoid
code duplication. Rename the "clamp" tests to vary_max_freq and
vary_min_freq.

v2: Fix compile warning
v3: Review comments (Ashutosh). Added a FIXME for the media RP0 case.
v4: Checkpatch (strict) fixes, remove FIXME and other comments (Ashutosh)

Fixes commit 8ee2c227 ("drm/i915/guc/slpc: Add SLPC selftest")

Cc: Ashutosh Dixit <ashutosh.dixit@intel.com>
Signed-off-by: Vinay Belgaumkar <vinay.belgaumkar@intel.com>
Reviewed-by: Ashutosh Dixit <ashutosh.dixit@intel.com>
Signed-off-by: John Harrison <John.C.Harrison@Intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220627230346.27720-1-vinay.belgaumkar@intel.com