drm/radeon: Allow panel preferred EDID to override BIOS native mode

We have two sources of information about panel capabilities on mobile
radeon - the BIOS, which gives us a native mode, and the panel's preferred
mode. In theory these two will always match, but there's some corner cases
where the BIOS hasn't been fully initialised and so the native mode in it
ends up with default values. However, if we get a panel with reasonable
EDID, it's probably the case that the panel's preferred mode does actually
represent the panel capabilities. This patch handles that case by replacing
the native mode with the panel's preferred mode if the resolutions don't
match. Systems without a valid internal panel EDID will still use the BIOS
native mode.

Signed-off-by: Matthew Garrett <mjg@redhat.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>