drm/i915/crtc: rename intel_get_crtc_for_pipe() to intel_crtc_for_pipe()

The "get" in the name implies reference counting, remove it. This also
makes the function conform to naming style.

Suggested-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/6105d0ff44efac3c999af6382e4b0729e251f1e1.1638366969.git.jani.nikula@intel.com