package/ti-gfx: do not forcefully remove source dir on extract

ti-gfx uses custom extract commands, which first and foremost removes
the source dir unconditionally.

Thee side-effect of this is that make will re-extract, re-patch and
probably re-build ti-gfx at each run.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Spenser Gilliland <spenser@gillilanding.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>