drm/tinydrm: make function st7586_pipe_enable static

The function st7586_pipe_enable  is local to the source
and does not need to be in global scope, so make it static.

Cleans up sparse warning:
symbol 'st7586_pipe_enable' was not declared. Should it be static?

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Acked-By: David Lechner <david@lechnology.com>
[noralf: fixed: Alignment should match open parenthesis]
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20170816092306.10969-1-colin.king@canonical.com