drm/amd: fix compiler error to support older compilers

‘for’ loop initial declarations are only allowed in C99 or C11 mode

Signed-off-by: Harish Kasiviswanathan <Harish.Kasiviswanathan@amd.com>
Reviewed-by: Mukul Joshi <mukul.joshi@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>