drm/amdgpu: Off by one in dm_dmub_outbox1_low_irq()

The > ARRAY_SIZE() should be >= ARRAY_SIZE() to prevent an out of bounds
access.

Fixes: e27c41d5 ("drm/amd/display: Support for DMUB HPD interrupt handling")
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>