drm/amd/display: restore edid reading from a given i2c adapter

When switching to drm_edid, we slightly changed how to get edid by
removing the possibility of getting them from dc_link when in aux
transaction mode. As MST doesn't initialize the connector with
`drm_connector_init_with_ddc()`, restore the original behavior to avoid
functional changes.

v2:
- Fix build warning of unchecked dereference (kernel test bot)

CC: Alex Hung <alex.hung@amd.com>
CC: Mario Limonciello <mario.limonciello@amd.com>
CC: Roman Li <Roman.Li@amd.com>
CC: Aurabindo Pillai <Aurabindo.Pillai@amd.com>
Fixes: 48edb2a4 ("drm/amd/display: switch amdgpu_dm_connector to use struct drm_edid")
Reviewed-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Melissa Wen <mwen@igalia.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>