omapdrm: fix the dependency to omapdss

omapdrm uses "select" in Kconfig to enable omapdss. This doesn't work
correctly, as "select" forces omapdss to be enabled in the config even
if it normally could not be enabled because of missing Kconfig
dependencies.

This causes a build break on ARM, when using allyesconfig:

drivers/video/omap2/dss/dss.c: In function 'dss_calc_clock_div':
drivers/video/omap2/dss/dss.c:572:20: error: 'CONFIG_OMAP2_DSS_MIN_FCK_PER_PCK' undeclared (first use in this function)
drivers/video/omap2/dss/dss.c:572:20: note: each undeclared identifier is reported only once for each function it appears in

Instead of using select, this patch changes omapdrm to use "depend
on".

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>