drm/amdgpu: Fix missing break in ATOM_ARG_IMM Case of atom_get_src_int()

Missing break statement in the ATOM_ARG_IMM case of a switch statement,
adds the missing break statement, ensuring that the program's control
flow is as intended.

Fixes the below:
drivers/gpu/drm/amd/amdgpu/atom.c:323 atom_get_src_int() warn: ignoring unreachable code.

Fixes: d38ceaf9

 ("drm/amdgpu: add core driver (v4)")
Cc: Jammy Zhou <Jammy.Zhou@amd.com>
Cc: Christian König <christian.koenig@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Srinivasan Shanmugam <srinivasan.shanmugam@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>