drm: add debug print to update_vblank_count

Since we are logging all vblank counter updates 30 lines below,
it is also good to have some details whether and how vblank count
difference is calculated.

Signed-off-by: Oleg Vasilev <oleg.vasilev@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190613121802.2193-1-oleg.vasilev@intel.com