drm/amdgpu: avoid out of bounds access on array interrupt_status_offsets

The check for an out of bound index into array interrupt_status_offsets
is off-by-one. Fix this and also don't compared to a hard coded array
size but use adev->mode_info.num_hpd instead.

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>