drm/radeon: Update radeon_get_vblank_counter_kms()

Commit 88e72717 ("drm/irq: Use unsigned int pipe in public API")
updated the prototype of this function but not the implementation. This
wasn't noticed even through compile tests because the prototype is part
of the source file that uses it and hence the compiler won't know the
prototype when it compiles the implementation.

The right thing would've been to move the prototype to a header that's
included in radeon_kms.c so that the implementation signature could be
checked against it, but the closest thing would've been radeon_drv.h
and including that results in a lot of build errors, so we'll leave it
as is for now.

Cc: Christian König <christian.koenig@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>