drm/i915: Use direction definition DMA_BIDIRECTIONAL instead of PCI_DMA_BIDIRECTIONAL

Replace direction definition PCI_DMA_BIDIRECTIONAL
with DMA_BIDIRECTIONAL, because it helps to enhance readability
and avoid possible inconsistency.

Signed-off-by: Cai Huoqing <caihuoqing@baidu.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210925124613.144-1-caihuoqing@baidu.com