drm/amdgpu/sdma5: fix number of sdma5 trap irq types for navi1x

v2: set num_types based on num_instances

navi1x has 2 sdma engines but commit
"e7b58d03b678 drm/amdgpu: reorganize sdma v4 code to support more instances"
changes the max number of sdma irq types (AMDGPU_SDMA_IRQ_LAST) from 2 to 8
which causes amdgpu_irq_gpu_reset_resume_helper() to recover irq of sdma
engines with following logic:

(enable irq for sdma0) * 1 time
(enable irq for sdma1) * 1 time
(disable irq for sdma1) * 6 times

as a result, after gpu reset, interrupt for sdma1 is lost.

Signed-off-by: Xiaojie Yuan <xiaojie.yuan@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>