Retry the dpcd fetch several times.  Some eDP panels
fail several times before the fetch is successful.

bug:
https://bugs.freedesktop.org/show_bug.cgi?id=73530

Ported from radeon.
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>