drm/panel: Provide convenience wrapper for .get_modes()

Add a convenience wrapper for the struct drm_panel_funcs' .get_modes()
function so that not every driver needs to check that the panel driver
implements the function before calling it.

Signed-off-by: Ajay Kumar <ajaykumar.rs@samsung.com>
[treding: extract from larger patch, commit message]
Signed-off-by: Thierry Reding <treding@nvidia.com>