i965/fs: fix FS_OPCODE_CINTERP for unpacked double input varyings

i965/fs: fix FS_OPCODE_CINTERP for unpacked double input varyings

Data starts at suboffet 3 in 32-bit units (12 bytes), so it is not
64-bit aligned and the current implementation fails to read the data
properly. Instead, when there is is a double input varying, read it as
vector of floats with twice the number of components.

Signed-off-by: Samuel Iglesias Gonsálvez <siglesias@igalia.com>
Reviewed-by: Timothy Arceri <timothy.arceri@collabora.com>
Cc: "12.0" <mesa-stable@lists.freedesktop.org>