drm/i915/guc/slpc: Add waitboost functionality for SLPC

Add helper in RPS code for handling SLPC and non-SLPC paths.
When boost is requested in the SLPC path, we can ask GuC to ramp
up the frequency req by setting the minimum frequency to boost freq.
Reset freq back to the min softlimit when there are no more waiters.

v2: Schedule a worker thread which can boost freq from within
an interrupt context as well.

v3: No need to check against requested freq before scheduling boost
work (Ashutosh)

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/20211102012608.8609-3-vinay.belgaumkar@intel.com