fbdev: msm: Avoid UAF in mdss_dsi_cmd_write

In mdss_dsi_cmd_write, a failure in copying the cmds to
'string_buf' can cause an early return. In this case,
the 'pcmds->string_buf' won't be pointing to a valid
buffer. This can lead to use-after-free and memory leak.
To avoid this, assign the newly allocated buffer to
'pcmds->string_buf' after returning from krealloc call.

Change-Id: I286f12c86078d1989cb09453c8a395a4ad94b324
Signed-off-by: Nirmal Abraham <nabrah@codeaurora.org>