OMAP4: DSS2: Rename hdmi_omap4_panel.c to hdmi_panel.c

As the panel driver will remain generic across OMAP's renaming it to
hdmi_panel.c

Signed-off-by: Mythri P K <mythripk@ti.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>