drm/radeon: avoid leaking edid data

commit 0ac66effe7fcdee55bda6d5d10d3372c95a41920 upstream.

In some cases we fetch the edid in the detect() callback
in order to determine what sort of monitor is connected.
If that happens, don't fetch the edid again in the get_modes()
callback or we will leak the edid.

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>