drm/amd/display: Use region6 size in fw_meta_info

[Why]
If driver allocated region6 size is not same as the size in firmware,
dmcub won't enable region6.

[How]
Use region6 size in dmcub_fw_meta instead of a constant value.

Reviewed-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Signed-off-by: JinZe Xu <jinze.xu@amd.com>
Signed-off-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>