drm/i915: Clean up encoder->crtc_mask setup

Use BIT(pipe) for better legibility when populating the crtc_mask
for encoders.

Also remove the redundant possible_crtcs setup for the TV encoder.

Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190708162048.4286-11-ville.syrjala@linux.intel.com