i965/nir: memleak during deserialization

Member nir of gl_program may be created without context.
So the object is has to be freed manually.
Issue observed with piglit-test:

	glsl-novertexdata -auto -fbo

Seems leak was happening in case:
 1. caching of shaders is enabled
 2. cache of shader wasn't generated previously or some
    error reading of it occurred

v2: solution updated (seems it is better do not deserialize
    nir if it already present due to only i965 constracts and
    uses it)

CC: Jordan Justen <jordan.l.justen@intel.com>
CC: Tapani Pälli <tapani.palli@intel.com>
Fixes: 1777c23a (i965: Add brw_program_deserialize_driver_blob)

Signed-off-by: Sergii Romantsov <sergii.romantsov@globallogic.com>
22 jobs for !967 with fix/meml_glsl2nir_nir_shader in 24 minutes and 42 seconds (queued for 2 seconds)
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