drm/amd/display: Fix an uninitialized index variable

clang points out that the new logic uses an always-uninitialized
array index:

drivers/gpu/drm/amd/amdgpu/../display/amdgpu_dm/amdgpu_dm.c:9810:38: warning: variable 'i' is uninitialized when used here [-Wuninitialized]
                        timing  = &edid->detailed_timings[i];
                                                          ^
drivers/gpu/drm/amd/amdgpu/../display/amdgpu_dm/amdgpu_dm.c:9720:7: note: initialize the variable 'i' to silence this warning

My best guess is that the index should have been returned by the
parse_hdmi_amd_vsdb() function that walks an array here, so do that.

Fixes: f9b4f20c ("drm/amd/display: Add Freesync HDMI support to DM")
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>