drm/radeon: Fix max_vblank_count value for current display engines

The value was much too low, which could cause the userspace visible
vblank counter to move backwards when the hardware counter wrapped
around.

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

diff --cc drivers/gpu/drm/radeon/radeon_irq_kms.c
Simple merge