drm/omap: gem: Fix misnamed and missing parameter descriptions

Fixes the following W=1 kernel build warning(s):

 drivers/gpu/drm/omapdrm/omap_gem.c:593: warning: Function parameter or member 'file' not described in 'omap_gem_dumb_create'
 drivers/gpu/drm/omapdrm/omap_gem.c:593: warning: Excess function parameter 'drm_file' description in 'omap_gem_dumb_create'
 drivers/gpu/drm/omapdrm/omap_gem.c:619: warning: Function parameter or member 'offset' not described in 'omap_gem_dumb_map_offset'

Cc: Tomi Valkeinen <tomi.valkeinen@ti.com>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Sumit Semwal <sumit.semwal@linaro.org>
Cc: "Christian König" <christian.koenig@amd.com>
Cc: Rob Clark <rob.clark@linaro.org>
Cc: dri-devel@lists.freedesktop.org
Cc: linux-media@vger.kernel.org
Cc: linaro-mm-sig@lists.linaro.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201106214949.2042120-9-lee.jones@linaro.org