drm/i915: Extract VESA DSC bpp alignment to separate function

We might to use that function separately from intel_dp_dsc_compute_config
for DP DSC over MST case, because allocating bandwidth in that
case can be a bit more tricky. So in order to avoid code copy-pasta
lets extract this to separate function and reuse it for both SST
and MST cases.

v2: Removed multiple blank lines
v3: Rename intel_dp_dsc_nearest_vesa_bpp to intel_dp_dsc_nearest_valid_bpp
    to reflect its meaning more properly.
    (Manasi Navare)

Signed-off-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Reviewed-by: Manasi Navare <manasi.d.navare@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221123100551.29080-1-stanislav.lisovskiy@intel.com