drm/i915/dp: Don't roundup max bpp, while computing compressed bpp

While computing compressed bpp, maximum value of bits_per_pixel is
calculated that can be supported with the given link configuration
for a given mode. Avoid rounding up of this max bits_per_pixel.
Also improve documentation for computing max bits_per_pixel.

Signed-off-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Reviewed-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Signed-off-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230223115509.3980226-1-ankit.k.nautiyal@intel.com