perf/x86/intel/uncore: Clean up client IMC

The client IMC block is accessed by MMIO. Current code uses an informal
way to access the block, which is not recommended.

Clean up the code by using __iomem annotation and the accessor
functions (read[lq]()).

Move exit_box() and read_counter() to generic code, which can be shared
with the server code later.

Signed-off-by: Kan Liang <kan.liang@linux.intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: acme@kernel.org
Cc: eranian@google.com
Link: https://lkml.kernel.org/r/1556672028-119221-6-git-send-email-kan.liang@linux.intel.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>