drm/amdgpu: fix a compiling error in old kernels

This patch is used to fix following compiling error that occurs in
some old kernels.

 error: ‘for’ loop initial declarations are only allowed in C99 mode
    for (int i = 0; i < dc->res_pool->res_cap->num_dsc; i++) {

Signed-off-by: Asher Song <Asher.Song@amd.com>
Reviewed-by: Guchun Chen <guchun.chen@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>