drm/i915/slpc: Let's fix the PCODE min freq table setup for SLPC

We need to inform PCODE of a desired ring frequencies so PCODE update
the memory frequencies to us. rps->min_freq and rps->max_freq are the
frequencies used in that request. However they were unset when SLPC was
enabled and PCODE never updated the memory freq.

v2 (as Suggested by Ashutosh): if SLPC is in use, let's pick the right
   frequencies from the get_ia_constants instead of the fake init of
   rps' min and max.

v3: don't forget the max <= min return

v4: Move all the freq conversion to intel_rps.c. And the max <= min
    check to where it belongs.

v5: (Ashutosh) Fix old comment s/50 HZ/50 MHz and add a doc explaining
    the "raw format"

Fixes: 7ba79a67

 ("drm/i915/guc/slpc: Gate Host RPS when SLPC is enabled")
Cc: <stable@vger.kernel.org> # v5.15+
Cc: Ashutosh Dixit <ashutosh.dixit@intel.com>
Tested-by: Sushma Venkatesh Reddy <sushma.venkatesh.reddy@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Ashutosh Dixit <ashutosh.dixit@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220831214538.143950-1-rodrigo.vivi@intel.com
(cherry picked from commit 018a7bdb

)
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>