drm/amd/display: Remove unnecessary amdgpu_irq_get/put

[WHY & HOW]
commit 7fb363c5 ("drm/amd/display: Let drm_crtc_vblank_on/off manage interrupts")
lets drm_crtc_vblank_* to manage interrupts in amdgpu_dm_crtc_set_vblank,
and amdgpu_irq_get/put do not need to be called here.  Part of that
patch got lost somehow, so fix it up.

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>