drm: Simplify return value of drm_get_last_vbltimestamp

Imo u32 hints at a register value, but in reality all callers only
care whether the sampled timestamp is precise or not. So give them
just a bool.

Also move the declaration out of drmP.h, it's only used in drm_irq.c.

v2: Also drop the EXPORT_SYMBOL, spotted by Mario.

Cc: Mario Kleiner <mario.kleiner.de@gmail.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Mario Kleiner <mario.kleiner.de@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>