drm/modes: Make sure to parse valid rotation value from cmdline

drm/modes: Make sure to parse valid rotation value from cmdline

A rotation value should have exactly one rotation angle.
At the moment there is no validation for this when parsing video=
parameters from the command line. This causes problems later on
when we try to combine the command line rotation with the panel
orientation.

To make sure that we generate a valid rotation value:
  - Set DRM_MODE_ROTATE_0 by default (if no rotate= option is set)
  - Validate that there is exactly one rotation angle set
    (i.e. specifying the rotate= option multiple times is invalid)

Signed-off-by: Stephan Gerhold <stephan@gerhold.net>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/20200117153429.54700-2-stephan@gerhold.net