drm/omap: dsi: Fix excessive stack usage

dsi_dump_dsi_irqs(), a function used for debugfs prints, has a large
struct in its frame, which can result in:

drivers/gpu/drm/omapdrm/dss/dsi.c:1126:1: warning: the frame size of 1060 bytes is larger than 1024 bytes [-Wframe-larger-than=]

As the performance of the function is of no concern, let's allocate the
struct with kmalloc instead.

Compile-tested only.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Reported-by: kernel test robot <lkp@intel.com>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20220916082206.167427-1-tomi.valkeinen@ideasonboard.com