drm/amd: Change `jpeg_v4_0_5_start_dpg_mode()` to void

jpeg_v4_0_5_start_dpg_mode() always returns 0 and the return value
doesn't get used in the caller jpeg_v4_0_5_start(). Modify the
function to be void.

Reported-by: coverity-bot <keescook+coverity-bot@chromium.org>
Addresses-Coverity-ID: 1583635 ("Code maintainability issues")
Fixes: 0a119d53

 ("drm/amdgpu/jpeg: add support for jpeg DPG mode")
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>