drm/bridge: lt8912b: Fix bridge_detach

The driver calls lt8912_bridge_detach() from its lt8912_remove()
function. As the DRM core detaches bridges automatically, this leads to
calling lt8912_bridge_detach() twice. The code probably has tried to
manage the double-call with the 'is_attached' variable, but the driver
never sets the variable to false, so its of no help.

Fix the issue by dropping the call to lt8912_bridge_detach() from
lt8912_remove(), as the DRM core will handle the detach call for us,
and also drop the useless is_attached field.

Fixes: 30e2ae94

 ("drm/bridge: Introduce LT8912B DSI to HDMI bridge")
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Reviewed-by: Robert Foss <rfoss@kernel.org>
Signed-off-by: Robert Foss <rfoss@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20230804-lt8912b-v1-1-c542692c6a2f@ideasonboard.com