drm/i915/guc/slpc: Check GuC status before freq boost

It's possible that i915 might get wedged between a boost
and un-boost. Validate the i915-GuC connection before trying
to send a H2G to change the min frequency.

Bug: https://gitlab.freedesktop.org/drm/intel/-/issues/4464



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/20211112071016.9640-1-vinay.belgaumkar@intel.com