drm/msm/dsi: One function call less in dsi_init() after error detection

The dsi_destroy() function was called in two cases by the dsi_init() function
during error handling even if the passed variable contained a null pointer.

* This implementation detail could be improved by adjustments for jump
  targets according to the Linux coding style convention.

* Drop an unnecessary initialisation for the variable "msm_dsi" then.

Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
[add couple missing ERR_PTR()'s]
Signed-off-by: Rob Clark <robdclark@gmail.com>