minui: fix mem leakage issue

Use free to release GRSurface object will not call data_'s
DataDeleter, then the buf data_ points to will leakage.
Use delete release GRSurface object.

Test: manual

Change-Id: I53a9ea5a4ad67de1662c707a5ea59d738bd9c2d8
Signed-off-by: lijiazi <lijiazi@xiaomi.com>