drm/tinydrm/mipi-dbi: Remove CMA helper dependency

mipi-dbi depends on the CMA helper through its use of
drm_fb_cma_get_gem_obj(). This is an unnecessary dependency to drag in for
drivers that only want to use the MIPI DBI interface part.
Avoid this by open coding the function.

Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Reviewed-by: David Lechner <david@lechnology.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190722104312.16184-6-noralf@tronnes.org