drm: atmel-hlcdc: use double rate for pixel clock only if supported

Doubled system clock should be used as pixel cock source only if this
is supported. This is emphasized by the value of
atmel_hlcdc_crtc::dc::desc::fixed_clksrc.

Fixes: a6eca2ab ("drm: atmel-hlcdc: add config option for clock selection")
Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Cc: Boris Brezillon <bbrezillon@kernel.org>
Cc: <stable@vger.kernel.org> # v5.3+
Link: https://patchwork.freedesktop.org/patch/msgid/1576672109-22707-2-git-send-email-claudiu.beznea@microchip.com