drm/omapdrm: Workaround for errata i734 (LCD1 Gamma) in DSS dispc

Workaround for errata i734 in DSS dispc
 - LCD1 Gamma Correction Is Not Working When GFX Pipe Is Disabled

For gamma tables to work on LCD1 the GFX plane has to be used at least
once after DSS HW has come out of reset. The workaround sets up a
minimal LCD setup with GFX plane and waits for one vertical sync irq
before disabling the setup and continuing with the context
restore. The physical outputs are gated during the operation.

For details see:
OMAP543x Multimedia Device Silicon Revision 2.0 Silicon Errata
Literature Number: SWPZ037E
Or some other relevant errata document for the DSS IP version.

Signed-off-by: Jyri Sarha <jsarha@ti.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>