drm/dp/mst: Validate REMOTE_I2C_READ harder

drm/dp/mst: Validate REMOTE_I2C_READ harder

Make sure i2c msgs we're asked to transfer conform to the
requirements of REMOTE_I2C_READ. We were only checking that the
last message is a read, but we must also check that the preceding
messages are all writes. Also check that the length of each
message isn't too long.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180928180403.22499-2-ville.syrjala@linux.intel.com
Reviewed-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>