MA-7026 fix out of vmalloc issue on sabreauto_6qp board

Context buffer size is larger on GPU gc2000+ comparing with gc2000
and gc880 as some new GPU features are enabled on gc2000+, we meet
out of vmalloc size issue when run monkey test with vmalloc=256M
on sabreauto_6qp(gc2000+) board, enlarge vmalloc size to 320M to
fix the issue.

Monkey test command:
while true;do monkey -v 100000;done

Kernel stack:
vmap allocation for size 610304 failed: use vmalloc=<size> to increase size.
vmalloc: allocation failure: 605184 bytes
RenderThread: page allocation failure: order:0, mode:0xd2
CPU: 2 PID: 1317 Comm: RenderThread Not tainted 3.14.38-05787-gd5e5aaa #1
[<c0015314>] (unwind_backtrace) from [<c0011c1c>] (show_stack+0x10/0x14)
[<c0011c1c>] (show_stack) from [<c0897e54>] (dump_stack+0x6c/0xc8) [<c0897e54>]
(dump_stack) from [<c00b7214>] (warn_alloc_failed+0xc8/0x10c) [<c00b7214>]
(warn_alloc_failed) from [<c00df844>] (__vmalloc_node_range+0x140/0x1ec)
[<c00df844>] (__vmalloc_node_range) from [<c00df92c>] (__vmalloc_node+0x3c/0x48)
[<c00df92c>] (__vmalloc_node) from [<c00df960>] (vmalloc+0x28/0x34)
[<c00df960>] (vmalloc) from [<c06053dc>] (gckOS_AllocateMemory+0x68/0x80)
:Dropped: keys=[<c06053dc>] (gckOS_AllocateMemory) from [<c0605410>]
(gckOS_Allocate+0x1c/0x2c) [<c0605410>] (gckOS_Allocate) from [<c061de0c>]
(gckCONTEXT_Update+0x61c/0x660) 26[<c061de0c>] (gckCONTEXT_Update) from [<c0611884>]
(gckCOMMAND_Commit+0x2c0/0xea0) [<c0611884>] (gckCOMMAND_Commit) from [<c060fef0>]
(gckKERNEL_Dispatch+0x8bc/0x11a8)  pointers=[<c060fef0>] (gckKERNEL_Dispatch) from [<c060bf80>]
(drv_ioctl+0x11c/0x2ac) [<c060bf80>] (drv_ioctl) from [<c010003c>] (do_vfs_ioctl+0x74/0x5c0)
116[<c010003c>] (do_vfs_ioctl) from [<c01005f0>] (SyS_ioctl+0x68/0x70)
trackballs=[<c01005f0>] (SyS_ioctl) from [<c000e560>] (ret_fast_syscall+0x0/0x30)

Signed-off-by: Richard Liu <r66033@freescale.com>