drm/amd/display: Remove redundant assignments

[WHAT & HOW]
log2_blk_height and log2_blk_width are assigned to 0 and then
immediately are updated to other values. The assignments to zero are
redudant and removed.

This fixes 18 UNUSED_VALUE issues reported by Coverity.

Reviewed-by: Rodrigo Siqueira <rodrigo.siqueira@amd.com>
Signed-off-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Rodrigo Siqueira <rodrigo.siqueira@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>