drm/amd/display: Move Mode Support Prefetch Checks To Its Own Function

[Why]
Large stack size observed in DCN4 mode support when compiling with clang.
Additional instrumentation added by compiler adds to stack size.
dml_core_mode_support ends up going over the stack size limit
due to the size of the function.

[How]
Move checks and calculations for prefetch to its own function.

Reviewed-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Austin Zheng <Austin.Zheng@amd.com>
Signed-off-by: Zaeem Mohamed <zaeem.mohamed@amd.com>
Tested-by: Mark Broadworth <mark.broadworth@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>