Use enum's type name to fix implicit conversion

Use enum's type name to fix implicit conversion

The implicit conversion happens because there is another enum defined with
the name GEN9_HEVC_ENC_SURFACE_TYPE introduced in the same commit 455351f9

To avoid the several warnings below (by clang), let's use the enum's
defined type name.

gen9_hevc_encoder.c:5754:34: warning: implicit conversion from
enumeration type 'enum _GEN9_HEVC_ENC_SURFACE_TYPE' to different
enumeration type 'enum GEN9_HEVC_ENC_SURFACE_TYPE' [-Wenum-conversion]
     HEVC_ENC_SURFACE_Y_2X, bti_idx,
     ^~~~~~~~~~~~~~~~~~~~~

Signed-off-by: Victor Toso <victortoso@redhat.com>