drm/i915: Extract drm_dp_atomic_find_vcpi_slots cycle to separate function

We would be using almost same code to loop through bpps while calling
drm_dp_atomic_find_vcpi_slots - lets remove this duplication by
introducing a new function intel_dp_mst_find_vcpi_slots_for_bpp

v2: Fix pbn_div calculation - shouldn't matter if its DSC or not.
v3: FIx rebase conflict, constant_n no longer needed.

Reviewed-by: Vinod Govindapillai <vinod.govindapillai@intel.com>
Signed-off-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221101094222.22091-4-stanislav.lisovskiy@intel.com