drm/i915/guc/slpc: Add get max/min freq hooks

Add helpers to read the min/max frequency being used
by SLPC. This is done by send a H2G command which forces
SLPC to update the shared data struct which can then be
read. These helpers will be used in a sysfs patch later
on.

v2: Address review comments (Michal W)
v3: Return err in case of query failure (Michal W)
v4: Move decode_min/max_freq to this patch

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/20210730202119.23810-9-vinay.belgaumkar@intel.com