drm: simpledrm: fix wrong unit with pixel clock

Pixel clock has to be set in kHz.

Signed-off-by: Alejandro Concepcion-Rodriguez <asconcepcion@acoro.eu>
Fixes: 11e8f5fd ("drm: Add simpledrm driver")
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/6f8554ef-1305-0dda-821c-f7d2e5644a48@acoro.eu