drm: atmel-hlcdc: enable sys_clk during initalization.

For SAM9X60 SoC, sys_clk is through lcd_gclk clock source and this
needs to be enabled before enabling lcd_clk.

Signed-off-by: Sandeep Sheriker Mallikarjun <sandeepsheriker.mallikarjun@microchip.com>
[claudiu.beznea@microchip.com: add fixed_clksrc checks]
Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/1556195748-11106-5-git-send-email-claudiu.beznea@microchip.com