drm/radeon: add UVD->DPM helper function (v5)

Add a helper function for counting the number of open stream handles.

v2: fix copy-pasta in comments and whitespace error
v3: make function static since it's only used in radeon_uvd.c
at the moment
v4: make non-static again for future changes
v5: make static again for new rework of dpm uvd changes

Signed-off-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>