drm: omapdrm: Rename occurrences of paddr to dma_addr

The fields, variables and functions deal with DMA addresses, name them
accordingly. The omap_gem_get_paddr() and omap_gem_put_paddr() will be
addressed differently separately.

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