drm/tinydrm: Rename variable mipi -> dbi

struct mipi_dbi is going to be split into an interface part and a display
pipeline part. The interface part can be used by drivers that need to
initialize the controller, but that won't upload the framebuffer over
this interface.

tinydrm uses the variable name 'mipi' but this is not a good name since
MIPI refers to a lot of standards. This patch changes the variable name
to 'dbi' where it refers to the interface part of struct mipi_dbi.

Functions that use both future parts will have both variables temporarily
pointing to the same structure.

Cc: Eric Anholt <eric@anholt.net>
Cc: David Lechner <david@lechnology.com>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Acked-by: David Lechner <david@lechnology.com>
Acked-by: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20190722104312.16184-3-noralf@tronnes.org