drm/amd/display: Fix wrong format specifier in amdgpu_dm.c

[Why]
Currently, the 32bit kernel build fails due to an incorrect string
format specifier. ARRAY_SIZE() returns size_t type as it uses sizeof().
However, we specify it in a string as %ld. This causes a compiler error
and causes the 32bit build to fail.

[How]
Change the %ld to %zu as size_t (which sizeof() returns) is an unsigned
integer data type. We use 'z' to ensure it also works with 64bit build.

Reviewed-by: Harry Wentland <Harry.Wentland@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Hayden Goodfellow <Hayden.Goodfellow@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>