drm/amd/display: tweak the kerneldoc for active_vblank_irq_count

Commit 71338cb4 ("drm/amd/display: enable idle optimizations for linux
(MALL stutter)") adds active_vblank_irq_count to amdgpu_display_manager
in ./drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.h.

The kerneldoc is incorrectly formatted, and make htmldocs warns:

  ./drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.h:
    340: warning: Incorrect use of kernel-doc format:          * @active_vblank_irq_count
    379: warning: Function parameter or member 'active_vblank_irq_count' not described in 'amdgpu_display_manager'

Tweak the kerneldoc for active_vblank_irq_count.

Reviewed-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>