drm/amdkfd: Fix MEC pipe interrupt enablement

for_each_inst modifies xcc_mask and therefore the loop doesn't
initialize properly interrupts on all pipes. Keep looping through xcc as
the outer loop to fix this issue.

Fixes: c4050ff1 ("drm/amdkfd: Use xcc mask for identifying xcc")
Signed-off-by: Lijo Lazar <lijo.lazar@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>