drm/amd/display: Fix out of bounds access on DNC31 stream encoder regs

[Why]
During dcn31_stream_encoder_create, if PHYC/D get remapped to F/G on B0
then we'll index 5 or 6 into a array of length 5 - leading to an
access violation on some configs during device creation.

[How]
Software won't be touching PHYF/PHYG directly, so just extend the
array to cover all possible engine IDs.

Even if it does by try to access one of these registers by accident
the offset will be 0 and we'll get a warning during the access.

Fixes: 2fe9a0e1 ("drm/amd/display: Fix DCN3 B0 DP Alt Mapping")
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>