drm/i915: Flatten intel_dp_check_mst_status() a bit

Make intel_dp_check_mst_status() somewhat legible by humans.

Note that the return value of drm_dp_mst_hpd_irq() is always
either 0 or -ENOMEM, and we never did anything with the latter
so we can just ignore the whole thing.

We can also get rid of the direct drm_dp_mst_topology_mgr_set_mst(false)
call since returning -EINVAL causes the caller to do the very same call
for us.

Cc: Lyude Paul <lyude@redhat.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200417152734.464-1-ville.syrjala@linux.intel.com
Reviewed-by: Lyude Paul <lyude@redhat.com>