drm: omapdrm: Rename GEM DMA sync functions

The omap_gem_cpu_sync() function operates at a page level, while the
omap_gem_dma_sync() function operates at a buffer level. Rename them to
omap_gem_cpu_sync_page() and omap_gem_dma_sync_buffer() respectively to
avoid confusion.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>