drm/mediatek/mtk_mipi_tx: checking the wrong variable

We should be checking "phy_provider" here not "phy".

Fixes: 2e54c14e ('drm/mediatek: Add DSI sub driver')
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Matthias Brugger <matthias.bgg@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20160701135934.GA15723@mwanda