drm/gma500: Fix end of loop tests for list_for_each_entry

The list_for_each_entry() iterator, "connector" in this code, can never be
NULL.  If we exit the loop without finding the correct  connector then
"connector" points invalid memory that is an offset from the list head.
This will eventually lead to memory corruption and presumably a kernel
crash.

Fixes: 9bd81acd ("gma500: Convert Oaktrail to work with new output handling")
Signed-off-by: Harshvardhan Jha <harshvardhan.jha@oracle.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210709073959.11443-1-harshvardhan.jha@oracle.com