i965: Fix calculation of layers array length for isl_view

i965: Fix calculation of layers array length for isl_view

Handle all cases in calculation of layers count for isl_view
taking into account texture view and image unit.
st_convert_image was taken as a reference.

When u->Layered is true the whole level is taken with respect to
image view. In other case only one layer is taken.

v3: (Józef Kucia and Ilia Mirkin)
    - Rewrote patch by taking st_convert_image as a reference
    - Removed now unused get_image_num_layers function
    - Changed commit message

v4: (Jason Ekstrand)
    - Added assert

Fixes: 5a8c8903
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=107856

Signed-off-by: Danylo Piliaiev <danylo.piliaiev@globallogic.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>