basesrc: avoid flush when starting
basesrc: avoid flush when starting

When we are doing the initial seek in startup, avoid doing a flush
(and unlock) because we know that the task is not started yet.

collects/gstreamer,krichter722/gstreamer,cfoch/gstreamer,jpxiong/gstreamer,jpxiong/gstreamer,cablelabs/gstreamer,cablelabs/gstreamer,collects/gstreamer,shelsonjava/gstreamer,jpakkane/gstreamer,lubosz/gstreamer,lubosz/gstreamer,krieger-od/gstreamer,lovebug356/gstreamer,Lachann/gstreamer,lubosz/gstreamer,krieger-od/gstreamer,krieger-od/gstreamer,justinjoy/gstreamer,centricular/gstreamer,cfoch/gstreamer,jpakkane/gstreamer,lovebug356/gstreamer,collects/gstreamer,Lachann/gstreamer,jpxiong/gstreamer,cablelabs/gstreamer,jpakkane/gstreamer,jpakkane/gstreamer,StreamUtils/gstreamer,mparis/gstreamer,centricular/gstreamer,StreamUtils/gstreamer,Lachann/gstreamer,lubosz/gstreamer,lovebug356/gstreamer,krichter722/gstreamer,cfoch/gstreamer,shelsonjava/gstreamer,cfoch/gstreamer,shelsonjava/gstreamer,justinjoy/gstreamer,Lachann/gstreamer,ensonic/gstreamer,krieger-od/gstreamer,krichter722/gstreamer,justinjoy/gstreamer,StreamUtils/gstreamer,mparis/gstreamer,ensonic/gstreamer,centricular/gstreamer,shelsonjava/gstreamer,centricular/gstreamer,centricular/gstreamer,ensonic/gstreamer,surround-io/gstreamer,mparis/gstreamer,collects/gstreamer,collects/gstreamer,justinjoy/gstreamer,ensonic/gstreamer,krichter722/gstreamer,lovebug356/gstreamer,cablelabs/gstreamer,justinjoy/gstreamer,cfoch/gstreamer,Lachann/gstreamer,StreamUtils/gstreamer,cablelabs/gstreamer,jpxiong/gstreamer,surround-io/gstreamer,surround-io/gstreamer,ensonic/gstreamer,surround-io/gstreamer,lovebug356/gstreamer,lubosz/gstreamer,jpakkane/gstreamer,shelsonjava/gstreamer,mparis/gstreamer,krichter722/gstreamer,krieger-od/gstreamer,jpxiong/gstreamer,mparis/gstreamer,StreamUtils/gstreamer,surround-io/gstreamer