drm/amdgpu/display: create fake mst encoders ahead of time (v4)

Prevents a warning in the MST create connector case.

v2: create global fake encoders rather per connector fake encoders
to avoid running out of encoder indices.

v3: use the actual number of crtcs on the asic rather than the max
to conserve encoders.

v4: v3 plus missing hunk I forgot to git add.

Bug: https://gitlab.freedesktop.org/drm/amd/-/issues/1108


Fixes: c6385e50 ("drm/amdgpu: drop legacy drm load and unload callbacks")
Reviewed-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org # 5.7.x