ANDROID: sched/fair: Cleanup cpu_util{_wake}()

The current implementation of cpu_util and cpu_util_wake makes more
difficult the backporting of mainline patches and it also has some
features not longer required by the current EAS code, e.g. delta
utilization in __cpu_util().

Let's clean up these functions definitions to:
1. get rid of the not longer required __cpu_util(cpu, delta)
   This function is now only called with delta=0 and thus we can refold
   its implementation into the original wrapper function cpu_util(cpu)
2. optimize for the WALT path on CONFIG_SCHED_WALT builds
   Currently indeed we execute some not necessary PELT related code even
   when WALT signals are required.
   Let's change this by assuming that on CONFIG_SCHED_WALT build we are
   likely using WALT signals. While on !CONFIG_SCHED_WALT we still have
   just the PELT signals with a code structure which matches mainline
3. move the definitions from sched/sched.h into sched/fair.c
   This is the only module using these functions and it also better
   align with the mainline location for these functions
4. get rid of the walt_util macro
   That macro has a function-like signature but it's modifying a
   parameter (passed by value) which makes it a bit confusing.
   Moreover, the usage of the min_t() macro to cap signals with
   capacity_orig_of() it makes not more required the explicit type cast
   used by that macro to support both 32 and 63 bits targets.
5. remove forward declarations
   Which are not required once the definition is moved at the top of
   fair.c, since they don't have other local dependencies.

Signed-off-by: Patrick Bellasi <patrick.bellasi@arm.com>
Change-Id: I61c9b7b8a0a34b494527c5aa76218c64543c16d2