DRI2: Refine the workaround for Mali r3p0 window resizing issue

Using the secure id 1 (framebuffer) to trick the Mali blob into
requesting DRI2 buffers again was not a very good idea. The problem
is that the blob still writes something there and corrupts the
framebuffer. So instead we try to assign secure id 2 to a dummy
4KiB UMP buffer allocated in memory and use it for the same purpose.

Signed-off-by: Siarhei Siamashka <siarhei.siamashka@gmail.com>