drm/panel-simple: When using panel-dpi, update desc

When using the "panel-dpi" compatible string, desc as passed
from DT is a dumy entry, and panel_dpi_probe allocates a new
one and attaches it to the panel.
However panel_simple_probe has already taken a local copy of
the variable, which means all the validation is done against
the empty dummy structure, not the configured data.

Update the local variable after panel_dpi_probe.

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>