The functions writing to the DISPC registers in apply.c are more complex
than needed. This patch cleans things up.

Two helper functions are added: need_isr(), which returns if an isr for
VSYNC is needed, and need_go(), which tells if there has been a write to
the shadow registers and we need to set the GO bit.

The functions that call the actual dispc write functions will set the
dirty and shadow_dirty flags themselves, instead of that being done on
the caller side. The writing functions also check if the dirty flag is
even set, and return immediately if not.

This allows us to clean up dss_write_regs().
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>