drm/modes: parse_cmdline: Explicitly memset the passed in drm_cmdline_mode struct

Instead of only setting mode->specified on false on an early exit and
leaving e.g. mode->bpp_specified and mode->refresh_specified as is,
lets be consistent and just zero out the entire passed in struct at
the top of drm_mode_parse_command_line_for_connector()

Changes in v3:
-Drop "mode->specified = false;" line instead of the "return false;" (oops)
This crasher was reported-by: kernel test robot <lkp@intel.com>

Acked-by: Maxime Ripard <mripard@kernel.org>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191118155134.30468-11-hdegoede@redhat.com