drm/radeon: Handle 64-bit return from drm_crtc_vblank_count()

570e8696

 ("drm: Widen vblank count to 64-bits [v3]") changed the
return type for drm_crtc_vblank_count() to u64. This could cause
potential problems if the return value is used in arithmetic operations
with a 32-bit reference HW vblank count. Explicitly typecasting this down
to u32 either fixes a potential problem or serves to add clarity in case
the implicit typecasting was already correct.

Cc: Keith Packard <keithp@keithp.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Reviewed-by: Keith Packard <keithp@keithp.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180203051302.9974-5-dhinakaran.pandiyan@intel.com