drm/i915: modify max_bw to return index to intel_bw_info

MTL uses the peak BW of a QGV point to lock the required QGV
point instead of the QGV index. Instead of passing the deratedbw
of the selected bw_info, return the index to the selected
bw_info so that either deratedbw or peakbw can be used based on
the platform.

v2: use idx to store index returned by max_bw_index functions

v3: return UINT_MAX in icl_max_bw_index in case no match found

v3: check idx >= ARRAY_SIZE

Signed-off-by: Vinod Govindapillai <vinod.govindapillai@intel.com>
Reviewed-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Signed-off-by: Radhakrishna Sripada <radhakrishna.sripada@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230606093509.221709-6-vinod.govindapillai@intel.com