drm/i915: Convert shared dpll reference count to a crtc mask

This will be used in a follow up patch to properly release shared DPLLs
without relying on the shared_dpll field in pipe_config.

v2: Fix white space error (Ville)
    Use hweight32() (Ville)

Signed-off-by: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>