drm/panel: Add prepare_upstream_first flag to drm_panel

Mapping to the drm_bridge flag pre_enable_upstream_first,
add a new flag prepare_upstream_first to drm_panel to allow
the panel driver to request that the upstream bridge should
be pre_enabled before the panel prepare.

Link: https://lore.kernel.org/all/a593a187fe3e6fc1ca5bf3db001ed87457e3d4ee.1646406653.git.dave.stevenson@raspberrypi.com/



Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>
Signed-off-by: Rahul T R <r-ravikumar@ti.com>