drm/amd/display: Delete a redundant memory setting in amdgpu_dm_irq_register_interrupt()

The memory was set to zero already by a call of the function “kzalloc”.
Thus remove an extra call of the function “memset” for this purpose.

This issue was detected by using the Coccinelle software.

Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>