drm/modes: Properly generate a drm_display_mode from a named mode

The framework will get the drm_display_mode from the drm_cmdline_mode it
got by parsing the video command line argument by calling
drm_connector_pick_cmdline_mode().

The heavy lifting will then be done by the
drm_mode_create_from_cmdline_mode() function.

In the case of the named modes though, there's no real code to make that
translation and we rely on the drivers to guess which actual display mode
we meant.

Let's modify drm_mode_create_from_cmdline_mode() to properly generate the
drm_display_mode we mean when passing a named mode.

Reviewed-by: Noralf Trønnes <noralf@tronnes.org>
Tested-by: Mateusz Kwiatkowski <kfyatek+publicgit@gmail.com>
Acked-in-principle-or-something-like-that-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://lore.kernel.org/r/20220728-rpi-analog-tv-properties-v10-9-256dad125326@cerno.tech


Signed-off-by: Maxime Ripard <maxime@cerno.tech>