drm/amd/display: Reduce stack size by splitting function

When compiling with allmodconfig, gcc highlights the following error:

drivers/gpu/drm/amd/amdgpu/../display/dc/dml2/display_mode_core.c: In function 'dml_core_mode_support':
drivers/gpu/drm/amd/amdgpu/../display/dc/dml2/display_mode_core.c:8229:1: error: the frame size of 2736 bytes is larger than 2048 bytes [-Werror=frame-larger-than=]
 8229 | } // dml_core_mode_support
      | ^
cc1: all warnings being treated as errors

This commit mitigates part of this problem by extracting the prefetch
code to its own function. After applying this commit, the stack size
reduces from 2736 to 2464, however, the stack size issue becomes part of
the new function.

Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: Roman Li <roman.li@amd.com>
Cc: Chaitanya Dhere <chaitanya.dhere@amd.com>
Fixes: 7966f319 ("drm/amd/display: Introduce DML2")
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>