[WHY]
Coverity reports OVERRUN issues within amdgpu_dm
interrupt registers. Do not check index value before
access array. Do not check NULL pointer.

[HOW]
Add index value check for array. Add check for
pointer from amdgpu_dm_irq_register_interrupt.
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Acked-by: Tom Chung <chiahsuan.chung@amd.com>
Signed-off-by: Hersen Wu <hersenxs.wu@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>