drm/radeon: use a fetch function to get the edid

We keep a cached version of the edid in radeon_connector which
we use for determining connectedness and when to enable certain
features like hdmi audio, etc.  When the user uses the firmware
interface to override the driver with some other edid the driver's
copy is never updated.  The fetch function will check if there
is a user supplied edid and update the driver's copy if there
is.

bug:
https://bugs.freedesktop.org/show_bug.cgi?id=80691



Signed-off-by: Alex Deucher <alexander.deucher@amd.com>