drm/mipi_dbi: Use simple right shift instead of double negation

GCC complains about dubious bitwise OR operand:

drivers/gpu/drm/drm_mipi_dbi.c:1024:49: warning: dubious: x | !y
  CC [M]  drivers/gpu/drm/drm_mipi_dbi.o

As long as buffer is consist of byte (u8) values, we may use
simple right shift and satisfy compiler. It also reduces amount of
operations needed.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Noralf Trønnes <noralf@tronnes.org>
Tested-by: Noralf Trønnes <noralf@tronnes.org>
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20191017114912.61522-1-andriy.shevchenko@linux.intel.com