drm/i915/dp: Iterate over output bpp with fractional step size

This patch adds support to iterate over compressed output bpp as per the
fractional step, supported by DP sink.

v2:
-Avoid ending up with compressed bpp, same as pipe bpp. (Stan)

Signed-off-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Reviewed-by: Suraj Kandpal <suraj.kandpal@intel.com>
Reviewed-by: Sui Jingfeng <suijingfeng@loongson.cn>
Link: https://patchwork.freedesktop.org/patch/msgid/20231110101020.4067342-7-ankit.k.nautiyal@intel.com