drm/amd/display: Reduce stack size in the mode support function

When we use the allmodconfig option we see the following error:

drivers/gpu/drm/amd/amdgpu/../display/dc/dml/dcn32/display_mode_vba_32.c: In function 'dml32_ModeSupportAndSystemConfigurationFull':
drivers/gpu/drm/amd/amdgpu/../display/dc/dml/dcn32/display_mode_vba_32.c:3799:1: error: the frame size of 2464 bytes is larger than 2048 bytes [-Werror=frame-larger-than=]
  3799 | } // ModeSupportAndSystemConfigurationFull

This commit fixes this issue by moving part of the mode support
operation from ModeSupportAndSystemConfigurationFull to a dedicated
function.

Cc: Harry Wentland <harry.wentland@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: Aurabindo Pillai <aurabindo.pillai@amd.com>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Tested-by: Stephen Rothwell <sfr@canb.auug.org.au>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>