drm/i915/slpc: Use platform limits for min/max frequency

GuC will set the min/max frequencies to theoretical max on
ATS-M. This will break kernel ABI, so limit min/max frequency
to RP0(platform max) instead.

Also modify the SLPC selftest to update the min frequency
when we have a server part so that we can iterate between
platform min and max.

v2: Check softlimits instead of platform limits (Riana)
v3: More review comments (Ashutosh)
v4: No need to use saved_min_freq and other comments (Ashutosh)

Bug: https://gitlab.freedesktop.org/drm/intel/-/issues/7030



Acked-by: Nirmoy Das <nirmoy.das@intel.com>
Reviewed-by: Riana Tauro <riana.tauro@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/20221024225453.4856-1-vinay.belgaumkar@intel.com