drm/edid: Fix a missing-check bug in drm_load_edid_firmware()

drm/edid: Fix a missing-check bug in drm_load_edid_firmware()

[ Upstream commit 9f1f1a2dab38d4ce87a13565cf4dc1b73bef3a5f ]

In drm_load_edid_firmware(), fwstr is allocated by kstrdup(). And fwstr
is dereferenced in the following codes. However, memory allocation
functions such as kstrdup() may fail and returns NULL. Dereferencing
this null pointer may cause the kernel go wrong. Thus we should check
this kstrdup() operation.
Further, if kstrdup() returns NULL, we should return ERR_PTR(-ENOMEM) to
the caller site.

Signed-off-by: Gen Zhang <blackgod016574@gmail.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190524023222.GA5302@zhanggen-UX430UQ
Signed-off-by: Sasha Levin <sashal@kernel.org>