drm/amd/display: Reduce stack size for dml21_ModeSupportAndSystemConfigurationFull

[Why & How]
With Werror enabled in the kernel we were failing the clang build since
dml21_ModeSupportAndSystemConfigurationFull's stack frame is 1064 when
building with clang, and exceeding the default 1024 stack frame limit.

The culprit seems to be the Pipe struct, so pull the relevant block
out into its own sub-function.

Signed-off-by: Harry Wentland <harry.wentland@amd.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>