drm/i2c: tda998x: don't read write-only registers

This patch takes care of the write-only registers of the tda998x.

The registers SOFTRESET, TBG_CNTRL_0 and TBG_CNTRL_1 have all bits
cleared after reset, so, they may be fully re-written.

The register MAT_CONTRL is set to
	MAT_CONTRL_MAT_BP | MAT_CONTRL_MAT_SC(1)
after reset, so, it may be fully set again to this value.

Tested-by: Russell King <rmk+kernel@arm.linux.org.uk>
Acked-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Jean-Francois Moine <moinejf@free.fr>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>