media: staging: imgu: do not hold spinlock during freeing mmu page table

ImgU need set the mmu page table in memory as uncached, and set back
to write-back when free the page table by set_memory_wb(),
set_memory_wb() can not do flushing without interrupt, so the spinlock
should not be hold during ImgU page alloc and free, the interrupt
should be enabled during memory cache flush.

This patch release spinlock before freeing pages table.

Signed-off-by: Bingbu Cao <bingbu.cao@intel.com>
Reviewed-by: Tomasz Figa <tfiga@chromium.org>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>