drm/omap: gem: Rename GEM function with omap_gem_* prefix

get_pages() as a local function name is too generic and easily confused
for a generic MM kernel function. Rename it to __omap_gem_get_pages().

Rename the is_contiguous(), is_cache_coherent(), evict(), evict_entry(),
fault_1d() and fault_2d() functions for the same reason.

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