drm: tegra: use crtc helper drm_crtc_from_index()

Function tegra_crtc_from_pipe() does the exactly same thing as what
crtc helper drm_crtc_from_index() provides.  Use the helper to save
some code.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Cc: Thierry Reding <thierry.reding@gmail.com>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: http://patchwork.freedesktop.org/patch/msgid/1483961145-18453-6-git-send-email-shawnguo@kernel.org