drm: provide a helper for the encoder possible_crtcs mask

The encoder possible_crtcs mask identifies which CRTCs can be bound to
a particular encoder.  Each bit from bit 0 defines an index in the list
of CRTCs held in the DRM mode_config crtc_list.  Rather than having
drivers trying to track the position of their CRTCs in the list, expose
the code which already exists for calculating the appropriate mask bit
for a CRTC.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Reviewed-by: David Herrmann <dh.herrmann@gmail.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
[treding@nvidia.com: add drm_crtc_index(), move to core]
Signed-off-by: Thierry Reding <treding@nvidia.com>