drm/nouveau/kms/nv50-: Share DP SST mode_valid() handling with MST

Currently, the nv50_mstc_mode_valid() function is happy to take any and
all modes, even the ones we can't actually support sometimes like
interlaced modes.

Luckily, the only difference between the mode validation that needs to
be performed for MST vs. SST is that eventually we'll need to check the
minimum PBN against the MSTB's full PBN capabilities (remember-we don't
care about the current bw state here). Otherwise, all of the other code
can be shared.

So, we move all of the common mode validation in
nouveau_connector_mode_valid() into a separate helper,
nv50_dp_mode_valid(), and use that from both nv50_mstc_mode_valid() and
nouveau_connector_mode_valid(). Note that we allow for returning the
calculated clock that nv50_dp_mode_valid() came up with, since we'll
eventually want to use that for PBN calculation in
nv50_mstc_mode_valid().

Signed-off-by: Lyude Paul <lyude@redhat.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>