drm/amdgpu/gfx10: Fix missing break in switch statement

Add missing break statement in order to prevent the code from falling
through to case AMDGPU_IRQ_STATE_ENABLE.

This bug was found thanks to the ongoing efforts to enable
-Wimplicit-fallthrough.

Fixes: a644d85a

 ("drm/amdgpu: add gfx v10 implementation (v10)")
Cc: stable@vger.kernel.org
Reviewed-by: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>