drm/radeon: Fix radeon_irq_kms_pflip_irq_get/put() imbalance

Fixes a regression in 3.16-rc1 compared to 3.15.

The unbalanced calls would presumably result in the page flip interrupts
never getting disabled once they are enabled.

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