drm/gma500: fix error path in gma_intel_setup_gmbus()

drm/gma500: fix error path in gma_intel_setup_gmbus()

The current code fails to call i2c_del_adapter on
dev_prev->gmbus[0].adapter, and if the for loop above failed already
at i==0, all hell breaks loose when we do the loop body for
i = -1,-2,...

Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Link: http://patchwork.freedesktop.org/patch/msgid/1455048677-19882-2-git-send-email-linux@rasmusvillemoes.dk
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>