sunxi-disp: Fix page alignment issues with UMP buffers

UMP buffers must be page aligned, otherwise we get a problem:

[   80.520000] UMP: ERR: drivers/gpu/mali/ump/common/ump_kernel_ref_drv.c
[   80.530000]            ump_dd_handle_create_from_phys_blocks()  50
[   80.530000] Trying to create UMP memory with unaligned size. Size: 5760000
[   80.540000]
[   80.540000] Unable to handle kernel NULL pointer dereference at virtual address 00000000
[   80.550000] pgd = ee09c000
[   80.560000] [00000000] *pgd=00000000
[   80.560000] Internal error: Oops: 5 [#1] PREEMPT ARM
[   80.560000] Modules linked in: disp_ump mali_drm drm mali ump 8192cu hdmi lcd disp cfbcopyarea cfbimgblt cfbfillrect
[   80.560000] CPU: 0    Not tainted  (3.4.24+ #30)
[   80.560000] PC is at ump_dd_secure_id_get+0xc/0x14 [ump]
[   80.560000] LR is at _disp_get_ump_secure_id+0x5c/0xec [disp_ump]
[   80.560000] pc : [<bf116c4c>]    lr : [<bf19905c>]    psr: 600f0013
[   80.560000] sp : ee4efd78  ip : ee4efd88  fp : ee4efd84
[   80.560000] r10: be8f9c4c  r9 : 00000000  r8 : bf03a4c4
[   80.560000] r7 : 00000001  r6 : bf03a4c4  r5 : 5a001000  r4 : eeb81000
[   80.560000] r3 : 00000000  r2 : 200f0093  r1 : 00000001  r0 : 00000000
[   80.560000] Flags: nZCv  IRQs on  FIQs on  Mode SVC_32  ISA ARM  Segment user
[   80.560000] Control: 10c5387d  Table: 6e09c019  DAC: 00000015

The code, which tries to split the framebuffer into two parts of equal
sizes for GET_UMP_SECURE_ID_BUF1 and GET_UMP_SECURE_ID_BUF2, may break
this assumption for certain screen resolutions (800x600 is particularly
poorly divisible by 4096). The user visible symptoms look like the X
server gets stuck and fails to start.

This patch addresses the problem in the following way:
1. Make sure that fix.smem_len is page aligned (like many of the
   other framebuffer drivers do).
2. If the splitting of framebuffer into two parts would result in
   unaligned second part, just disable double buffering.
3. Avoid wrapping the physical memory beyond the framebuffer
   for GET_UMP_SECURE_ID_BUF2 in the case if double buffering
   is not used by setting the size of UMP buffer to 0.

The problem has been reported at:
  https://groups.google.com/d/msg/linux-sunxi/kZNl9bFdE-g/4k8z2_i4nJgJ

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